Michael Mann, SciAm: Why Global Warming Will Cross a Dangerous Threshold in 2036

Emitting carbon dioxide at current rates will soon push Earth’s temperature up by 2 degrees Celsius. Here’s how to make the calculation yourself

by Michael E. Mann, Scientific American, March 18, 2014 (see also April edition)

If the world continues to burn fossil fuels at the current rate, global warming will rise 2 degrees Celsius by 2036, crossing a threshold that many scientists think will hurt all aspects of human civilization: food, water, health, energy, economy and national security. In my article "False Hope" in the April 2014 Scientific American, I reveal dramatic curves that show why the world will reach this temperature limit so quickly, and also why the recent slowdown in the rate of temperature increase, if it continues, will only buy us another 10 years.

These numbers come from calculations made by me and several colleagues. We plugged values of Earth’s “equilibrium climate sensitivity”—a common measure of the heating effect of greenhouse gases—into a so-called energy balance model. Scientists use the model to investigate possible climate scenarios.

You can try this exercise yourself. The text below explains the variables and steps involved. You can download the climate data here and the model code here. And you can compare your results with mine, which are here. You can also change the variables to see what other future scenarios are possible. One note: the model runs on MatLab software, which can be obtained here.

We employed a simple zero-dimensional Energy Balance Model (“EBM”—see references 1 through 5 below) of the form

C dT/dt = S(1-a)/4 + F_GHG -A-B T + w(t)      

to model the forced response of the climate to estimate natural and anthropogenic radiative forcing.

T is the temperature of Earth’s surface (approximated as the surface of a 70-meter-depth, mixed-layer ocean covering 70% of Earth’s surface area).  C = 2.08 x 10⁸ J K^-1m^-2 and is an effective heat capacity that accounts for the thermal inertia of the mixed-layer ocean, but does not allow for heat exchange with the deep ocean as in more elaborate “upwelling-diffusion models” (ref. 6). S ≈ 1370 Wm^-2 is the solar constant, and a ≈ 0.3 is the effective surface albedo. F_GHG is the radiative forcing by greenhouse gases, and w(t)  represents random weather effects, which was set to zero to analyze the pure radiative forced response.

The linear “gray body” approximation (ref. 3) LW = A+B T was used to model outgoing longwave radiation in a way that accounts for the greenhouse effect. The choice A = 221.3 WK^-1m^-2 and B = 1.25 Wm^-2 yields a realistic preindustrial global mean temperature T = 14.8 ^oC and an equilibrium climate sensitivity (ECS) of  DT_2xCO2 = 3.0 ^oC, consistent with midrange estimates by the International Panel on Climate Change (ref. 7).  B can be varied to change the ECS of the EBM. For example, the higher value B = 1.5 Wm^-2 yields a more conservative ECS of  DT_2xCO2= 2.5 ^oC.

Energy Balance Model Simulations

Historical Simulations. The model was driven with estimated annual natural and anthropogenic forcing over the years A.D. 850 to 2012. Greenhouse radiative forcing was calculated using the approximation (ref. 8) F_GHG = 5.35log(CO₂e/280), where 280 parts per million (ppm) is the preindustrial CO₂ level and CO₂e is the “equivalent” anthropogenic CO₂. We used the CO₂ data from ref. 9, scaled to give CO₂e values 20 percent larger than CO₂ alone (for example, in 2009 CO₂ was 380 ppm whereas CO₂e was estimated at 455 ppm). Northern Hemisphere anthropogenic tropospheric aerosol forcing was not available for ref. 9 so was taken instead from ref. 2, with an increase in amplitude by 5 percent to accommodate a slightly larger indirect effect than in ref. 2, and a linear extrapolation of the original series (which ends in 1999) to extend though 2012.

Estimated past changes in solar irradiance were prescribed as a change in the solar constant S whereas forcing by volcanic aerosols was prescribed as a change in the surface albedo a. Solar and volcanic forcing were taken from the General Circulation Model (GCM) simulation of ref. 3 described in the section above, with the following modifications: (1) solar forcing was rescaled under the assumption of a 0.1 percent change from Maunder Minimum to present, more consistent with recent estimates (ref. 9); (2) volcanic forcing was applied as the mean of the latitudinally varying volcanic forcing of ref. 9; (3) values for both series were updated through 2012.

Future Projections: For the purpose of the “business as usual” future projections, we have linearly extrapolated the CO₂ radiative forcing forward to 2100, based on the trend over the past decade (which is roughly equivalent, from a radiative forcing standpoint, to a forward projection of the exponential historical trajectory of CO2 emissions). We assume constant solar output, and assume no climatically significant future volcanic eruptions.

We have assumed that tropospheric aerosols decrease exponentially from their current values with a time constant of 60 years. This gives a net anthropogenic forcing change from 2000 to 2100 of 3.5 Wm^-2, roughly equivalent to the International Panel on Climate Change’s 5^th assessment report “RCP6” scenario, a future emissions scenario that assumes only modest efforts at mitigation.

Stabilization Scenarios: For the stabilization scenarios, we relax (with a 20-year time constant) the CO₂ concentration to a maximum specified value at 2100. We considered cases both where the anthropogenic tropospheric aerosol burden is assumed to (a) decrease exponentially from their current values with a time constant of 60 years, as in the future projections discussed in the previous section, and alternatively (b) remain constant at its current value.

Additional Details. Sensitivity analyses of the historical simulations were performed by Mann et al 2012 (ref.  4) with respect to (i) the equilibrium climate sensitivity assumed (varied from  DT_2xCO2 = 2-4 ^oC); the (ii) solar scaling (0.25 percent in place of 0.1 percent Maunder Minimum to present change); (iii) the volcanic aerosol loading estimates used; and (iv) the scaling of volcanic radiative forcing with respect to aerosol loading to account for possible size distribution effects. All of the alternative choices described above were found to yield qualitatively similar results.

Matlab source code for the energy balance model, data used in the calculations and the simulation results discussed in the Scientific American article are available at: www.meteo.psu.edu/holocene/public_html/supplements/EBMProjections

http://www.scientificamerican.com/article/mann-why-global-warming-will-cross-a-dangerous-threshold-in-2036/

Justin Gillis, NYT: Scientists Sound Alarm on Climate

A tumbleweed-covered field in drought-plagued California. Credit David McNew/Getty Images        

by Justin Gillis, "By Degrees," The New York Times, March 18, 2014

Early in his career, a scientist named Mario J. Molina was pulled into seemingly obscure research about strange chemicals being spewed into the atmosphere. Within a year, he had helped discover a global environmental emergency, work that would ultimately win a Nobel Prize.

Now, at 70, Dr. Molina is trying to awaken the public to an even bigger risk. He is spearheading a committee of the American Association for the Advancement of Science, the world’s largest general scientific society, which will release a stark report Tuesday on global warming.

The report will warn that the effects of human emissions of heat-trapping gases are already being felt, that the ultimate consequences could be dire, and that the window to do something about it is closing.

“The evidence is overwhelming: Levels of greenhouse gases in the atmosphere are rising,” says the report, which was made available early to The New York Times. 

“Temperatures are going up. Springs are arriving earlier. Ice sheets are melting. Sea level is rising. The patterns of rainfall and drought are changing. Heat waves are getting worse, as is extreme precipitation. The oceans are acidifying.”

In a sense, this is just one more report about global warming in a string going back decades. For anybody who was already paying attention, the report contains no new science. But the language in the 18-page report, called “What We Know,” is sharper, clearer and more accessible than perhaps anything the scientific community has put out to date.

And the association does not plan to stop with the report. The group, with a membership of 121,200 scientists and science supporters around the world, plans a broad outreach campaign to put forward accurate information in simple language.

The scientists are essentially trying to use their powers of persuasion to cut through public confusion over this issue.

Polls show that most Americans are at least somewhat worried about global warming. But people generally do not understand that the problem is urgent — that the fate of future generations (not necessarily that far in the future) is being determined by emission levels now. Moreover, the average citizen tends to think there is more scientific debate about the basics than there really is.

The report emphasizes that the experts have come to a consensus, with only a few dissenters. “Based on well-established evidence, about 97% of climate scientists have concluded that human-caused climate change is happening,” it says.

That is not the same as claiming that all questions about climate change have been answered. In fact, enormous questions remain, and the science of global warming entails a robust, evolving discussion.

The new report walks through a series of potential consequences of planetary warming, without asserting that any is sure to happen. They are possibilities, not certainties, and the distinction is crucial for an intelligent public debate about what to do. (The document and supporting material can be found on Tuesday at whatweknow.aaas.org.) The worst-case forecasts include severe food shortages as warming makes it harder to grow crops; an accelerating rise of the sea that would inundate coastlines too rapidly for humanity to adjust; extreme heat waves, droughts and floods; and a large-scale extinction of plants and animals.

“What’s extremely clear is that there’s a risk, a very significant risk,” Dr. Molina said by telephone from Mexico, where he spends part of his time. “You don’t need 100% certainty for society to act.”

Some of the scientists on Dr. Molina’s committee like to point out that people can be pretty intelligent about managing risk in their personal lives. It is unlikely that your house will burn down, yet you spend hundreds of dollars a year on insurance. When you drive to work in the morning, the odds are low that some careless driver will slam into you, but it is possible, so we have spent tens of billions of dollars putting seat belts and air bags in our cars.

The issue of how much to spend on lowering greenhouse gases is, in essence, a question about how much insurance we want to buy against worst-case outcomes. 

Scientists cannot decide that for us — and the report recognizes that by avoiding any specific recommendations about what to do. But it makes clear that lowering emissions, by some means, is the only way to lower the risks. Because so many people are confused about the science, the nation has never really had a frank political discussion about the options.

Only a few decades ago, the world confronted a similar question regarding chemicals called chlorofluorocarbons, then common in refrigerators, air-conditioners, cans of hair spray and deodorant.

At a Fort Lauderdale, Fla., conference in 1972, a California scientist named F. Sherwood Rowland learned that they were accumulating in the air. What, he wondered, would happen to them? He eventually put a young researcher in his laboratory, Dr. Molina, onto the question.

To their own shock, the team figured out that the chemicals would break down the ozone layer, a blanket of gas high above the ground that protects the world from devastating levels of ultraviolet radiation. As the scientific evidence of a risk accumulated, the public demanded action — and eventually got it, in the form of a treaty phasing out the compounds.

Global warming has been much harder to understand, not least because of a disinformation campaign financed by elements of the fossil-fuel industry.

But the new report is a recognition among scientists that they bear some responsibility for the confusion — that their well-meaning attempts to convey all the nuances and uncertainties of a complex field have obscured the core message about risks. The report reflects their resolve to try again, by clearing the clutter.

Will the American people hear the message this time?

http://www.nytimes.com/2014/03/18/science/scientists-sound-alarm-on-climate.html

Financial Times: Climate Chaos

Why the world faces climate chaos

 by Martin Wolf, Financial Times, May 15, 2013

We will watch the rise in greenhouse gases until it is too late to do anything about it

Last week the concentration of carbon dioxide in the atmosphere was reported to have passed 400 parts per million for the first time in 4.5 million years. It is also continuing to rise at a rate of about 2 ppm every year. On the present course, it could be 800 ppm by the end of the century. Thus, all the discussions of mitigating the risks of catastrophic climate change have turned out to be empty words.

Collectively, humanity has yawned and decided to let the dangers mount. Professor Sir Brian Hoskins, director of the Grantham Institute for Climate Change at Imperial College in London, notes that when the concentrations were last this high, “the world was warmer on average by three or four degrees Celsius than it is today. There was no permanent ice sheet on Greenland, sea levels were much higher, and the world was a very different place, although not all of these differences may be directly related to CO2 levels.”

His caveat is proper. Nonetheless, the greenhouse effect is basic science: it is why the earth has a more pleasant climate than the moon. CO2 is a known greenhouse gas. There are positive feedback effects from rising temperatures, via, for example, the quantity of water vapour in the atmosphere. In brief, humanity is conducting a huge, uncontrolled and almost certainly irreversible climate experiment with the only home it is likely to have. Moreover, if one judges by the basic science and the opinions of the vast majority of qualified scientists, risk of calamitous change is large.

What makes the inaction more remarkable is that we have been hearing so much hysteria about the dire consequences of piling up a big burden of public debt on our children and grandchildren. But all that is being bequeathed is financial claims of some people on other people. If the worst comes to the worst, a default will occur. Some people will be unhappy. 

But life will go on. Bequeathing a planet in climatic chaos is a rather bigger concern. There is nowhere else for people to go and no way to reset the planet’s climate system. If we are to take a prudential view of public finances, we should surely take a prudential view of something irreversible and much costlier.

So why are we behaving like this?

The first and deepest reason is that, as the civilisation of ancient Rome was built on slaves, ours is built on fossil fuels. What happened in the beginning of the 19th century was not an “industrial revolution” but an “energy revolution.” Putting carbon into the atmosphere is what we do. As I have argued in Climate Policy, what used to be the energy-intensive lifestyle of today’s high-income countries has gone global. Economic convergence between emerging and high-income countries is increasing demand for energy faster than improved energy efficiency is reducing it. Not only aggregate CO2 emissions but even emissions per head are rising. The latter is partly driven by China’s reliance on coal-powered electricity generation. (See charts.)

A second reason is opposition to any interventions in the free market. Some of this, no doubt, is driven by narrowly economic interests. But do not underestimate the power of ideas. To admit that a free economy generates a vast global external cost is to admit that the large-scale government regulation so often proposed by hated environmentalists is justified. For many libertarians or classical liberals, the very idea is unsupportable. It is far easier to deny the relevance of the science.

A symptom of this is clutching at straws. It is noted, for example, that average global temperatures have not risen recently, though they are far higher than a century ago. Yet periods of falling temperature within a rising trend have occurred before.

A third reason may be the pressure of responding to immediate crises that has consumed almost all the attention of policy makers in the high-income countries since 2007.

A fourth is a touching confidence that, should the worst comes to the worst, human ingenuity will find some clever ways of managing the worst results of climate change.

A fifth is the complexity of reaching effective and enforceable global agreements on the control of emissions among so many countries. Not surprisingly, the actual agreements reached give more an appearance of action than a reality.

A sixth is indifference to the interests of people to be born in a relatively distant future. As the old line goes: “Why should I care about future generations? What have they ever done for me?”

A final (and related) reason is the need to strike a just balance between poor countries and rich ones and between those who emitted most of the greenhouse gases in the past and those who will emit in the future.

The more one thinks about the challenge, the more impossible it is to envisage effective action. We will, instead, watch the rise in global concentrations of greenhouse gases. If it turns out to lead to a disaster, it will by then be far too late to do anything much about it.

So what might shift such a course? My view is, increasingly, that there is no point in making moral demands. People will not do something on this scale because they care about others, even including their own more remote descendants. They mostly care rather too much about themselves for that.

Most people believe today that a low-carbon economy would be one of universal privation. They will never accept such a situation. This is true both of the people of high-income countries, who want to retain what they have, and the people of the rest of the world, who want to enjoy what the people of high-income countries now have. A necessary, albeit not sufficient condition, then, is a politically sellable vision of a prosperous low-carbon economy. That is not what people now see. Substantial resources must be invested in the technologies that would credibly deliver such a future.

Yet that is not all. If such an opportunity does appear more credible, institutions must also be developed that can deliver it.

Neither the technological nor the institutional conditions exist at present. In their absence, there is no political will to do anything real about the process driving our experiment with the climate. Yes, there is talk and wringing of hands. But there is, predictably, no effective action. If that is to change, we must start by offering humanity a far better future. Fear of distant horror is not enough.

martin.wolf@ft.com

http://www.ft.com/cms/s/0/c926f6e8-bbf9-11e2-a4b4-00144feab7de.html

Climate change: Driving straight into catastrophe

Climate change: Driving straight into catastrophe

Posted by Jim at Monday, January 24, 2011

by Julio Godoy, desdemonadespair, January 24, 2011

PARIS (IPS) - Despite repeated warnings by environmental and climate experts that reduction of fossil fuel consumption and greenhouse gas emissions is fundamental to forestalling global warming, disaster appears imminent. According to the latest statistics, unprecedented climate change has Earth hurtling down a path of catastrophic proportions.

The Paris-based International Energy Agency (IEA) estimates that the global consumption of primary energy in 2010 reached some 500 exajoules (EJ), a number just under the worst-case scenario formulated ten years ago by the Intergovernmental Panel on Climate Change (IPCC). The IPCC’s Special Report on Emissions Scenarios, published in 2000, calculated the worst-case scenario as 525 EJ consumed in one calendar year.

The IEA found that coal was one of the largest sources of energy consumed in 2010, comprising approximately 27 percent of the total energy consumption. Coal, one of the cheapest sources of energy, is considered the filthiest of all, as far as greenhouse gases emissions (GHGE) are concerned.

Correspondingly, the global GHGE, measured as equivalent to carbon dioxide, reached at least 32 billion tonnes last year, only one step below the most pessimistic scenario imagined by the IPCC in 2000: 33 billion tonnes of CO2.

The results for 2010 were conditioned by the present global economic crisis – meaning that under normal economic circumstances, the numbers would have been higher. In other words, total consumption of energy in 2010 would have been worse than the most pessimistic scenario the IPCC formulated ten years ago had the global economy been in better shape.

These findings have prompted leading environmental experts to warn that humankind is racing towards destruction.

"The year 2010 was the hottest ever measured since the beginning of the recordings, 130 years ago," Anders Levermann, professor of climate system dynamics at the Physics Institute of the Potsdam University told IPS. …

Levermann explained that, contrary to appearance, the arctic winter in Western Europe is just another negative consequence of climate change.

"Global warming is melting the ice in the Kara Sea, in the Arctic Ocean," he explained. "This leads to a high pressure area above Siberia, which drives extremely cold winds towards Europe."

Levermann pointed out that the extreme global weather conditions experienced in 2010 -- very cold weather in Western Europe during the winter, massive floods in Pakistan and Australia, extremely hot summers in Russia and Western Europe -- illustrate the limits of even the most expert climate predictions.

"The more greenhouse gases we emit, the more the global climate gets out of control," Levermann said. "But the weather extremes that we cannot predict, such as the floods in Pakistan and Australia and the fires in Russia, are the ones that set the limits to human life." …

Levermann compared the consequences of global warming to a wall hidden in fog. "We cannot see the wall, but it is there. And we are driving at the highest possible speed towards it."

Link: http://www.desdemonadespair.net/2011/01/climate-change-driving-straight-into.html 

Must-read Hansen and Sato draft paper: We are at a climate tipping point that, once crossed, enables multi-meter sea level rise this century

Must-read Hansen and Sato draft paper: We are at a climate tipping point that, once crossed, enables multi-meter sea level rise this century

Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century

by Joseph Romm, Climate Progress, January 20, 2011

Right now, we’re headed towards an ice-free planet.  That takes us through the Eemian interglacial period of about 130,000 years ago when sea levels were 15 to 20 feet higher, when temperatures had been thought to be about 1 °C warmer than today.  Then we go back to the “early Pliocene, when sea level was about 25 m [82 feet] higher than today,” as NASA’s James Hansen and Makiko Sato explain in a new draft paper, “Paleoclimate Implications for Human-Made Climate Change.”

The question is how much warmer was it in the Eemian and early Pliocene than today — and how fast can the great ice sheets disintegrate?

We already know we’re at CO2 levels that risk catastrophe if they are sustained or exceeded for any extended period of time (see Science: CO2 levels haven’t been this high for 15 million years, when it was 5 to 10 °F warmer and seas were 75 to 120 feet higher).

Hansen and Sato go further, saying we’re actually at or very near the highest temperatures of the current Holocene interglacial — the last 12,000 years of relatively stable climate that has made modern civilization possible.

They argue that the Eemian was warmer than the Holocene maximum by “at most by about 1 °C, but probably by only several tenths of a degree Celsius.”  They make the remarkable finding that sea level rise will be highly nonlinear this century on our current business-as-usual [BAU] emissions that:

BAU scenarios result in global warming of the order of 3-6 °C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.

While this conclusion takes them well outside of every other recent prediction of sea level rise (SLR), Hansen deserves to be listened to because he has been right longer than almost anyone else in the field (see “Right for three decades: 1981 Hansen study finds warming trend that could raise sea levels“).   Also, at least one recent study that attempts to integrate a linear historically-based analysis with a rapid response term finds we are headed towards SLR of “as much as 1.9 metres (6 ft., 3 in.) by 2100″ if we stay on BAU (see “Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100“).

Hansen and Sato make their case for a strong nonlinear SLR based on a “phase change feedback mechanism,” that, as we’ll see, appears consistent with the recent scientific literature and observations:

There is a simple explanation for why the Eemian and Holsteinian were only marginally warmer than the Holocene and yet had (both) poles several degrees Celsius warmer. Earth at peak Holocene temperature is poised such that additional warming instigates large amplifying high-latitude feedbacks. Mechanisms on the verge of being instigated include loss of Arctic sea ice, shrinkage of the Greenland ice sheet, loss of Antarctic ice shelves, and shrinkage of the Antarctic ice sheets. These are not runaway feedbacks, but together they strongly amplify the impacts in polar regions of a positive (warming) climate forcing. 

Augmentation of peak Holocene temperature by even 1 °C would be sufficient to trigger powerful amplifying polar feedbacks, leading to a planet at least as warm as in the Eemian and Holsteinian periods, making ice sheet disintegration and large sea level rise inevitable. 

Empirical evidence supporting these assertions abounds. Global temperature increased 0.5 °C in the past three decades (Hansen et al., 2010) to a level comparable to the prior Holocene maximum, or a few tenths of a degree higher. Satellite observations reveal rapid reduction of Arctic sea ice (Stroeve et al., 2007) and surface melt on a large growing portion of the Greenland ice sheet (Steffen et al., 2004; Tedesco et al., 2011). 

Arctic response to human-made climate forcing is more apparent than Antarctic change, because the response time is quicker due to the large proportion of land area and Greenland’s temperature, which allows a large expansion of the area with summer melting. 

However, we must expect ice sheet mass balance changes will occur simultaneously in both hemispheres. Why? Because ice sheets in both hemispheres were in near-equilibrium with Holocene temperatures. That is probably why both Greenland and Antarctica began to shed ice in the past decade or so, because global temperature is just rising above the Holocene level. 

Ice sheet disintegration in Antarctica depends on melting the underside of ice shelves as the ocean warms, a process well underway at the Pine Island glacier (Scott et al., 2009). The glacier’s grounding line has retreated inland by tens of kilometers (Jenkins et al., 2010), and thinning of the ice sheet has spread inland hundreds of kilometers (Wingham et al., 2009).

The article has a longer discussion of the ‘albedo flip’ underlying their conclusion:

Summer melting on lower reaches of the ice sheets and on ice shelves introduces the “albedo flip” mechanism (Hansen et al., 2007). This phase change of water causes a powerful local feedback, which, together with moderate global warming, can substantially increase the length of the melt season. Such increased summer melting has an immediate local temperature effect, and it also will affect sea level, on a time scale that is being debated, as discussed below. 

We suggest that the warmest interglacials in the past 450,000 years were warm enough to bring the “albedo flip” phenomenon into play, while interglacials in the earlier part of the 800,000 year ice core record were too cool for surface melt on the Greenland and Antarctic ice sheets and ice shelves to be important. Increased surface melting, loss of ice shelves, and reduction of summer and autumn sea ice around the Antarctic and Greenland continents during the warmest interglacials would have a year-round effect on temperature, because the increased area of open water has its largest impact on surface air temperature in the cool seasons. 

Further, we suggest that the stability of sea level during the Holocene is a consequence of the fact that global temperature remained just below the level required to initiate the “albedo flip” mechanism on Greenland and West Antarctica. 

One implication of this interpretation is that the world today is on the verge of a level of global warming for which the equilibrium surface air temperature response on the ice sheets will exceed the global mean temperature increase by much more than a factor of two.

Coincidentally, a new article in Nature Geoscience, “Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008,” appears to lend support to this thesis.  After “synthesizing a variety of remote sensing and field measurements,” the authors find “the albedo feedback from the Northern Hemisphere cryosphere” is “substantially larger than comparable estimates obtained from 18 climate models.”  The news release notes:

A new analysis of the Northern Hemisphere’s “albedo feedback” over a 30-year period concludes that the region’s loss of reflectivity due to snow and sea ice decline is more than double what state-of-the-art climate models estimate. 

The findings are important, researchers say, because they suggest that Arctic warming amplified by the loss of reflectivity could be even more significant than previously thought.

Also, the Hansen-Sato thesis seems consistent with a 2008 study in Geophysical Research Letters by leading tundra experts, “Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss.” The lead author is David Lawrence of the National Center for Atmospheric Research (NCAR), who I have interviewed a number of times. The study’s ominous conclusion:

We find that simulated western Arctic land warming trends during rapid sea ice loss are 3.5 times greater than secular 21st century climate-change trends. The accelerated warming signal penetrates up to 1500 km inland…

See also “What exactly is polar amplification and why does it matter?“

Back to Hansen-Sato.  They have extended discussion of “linear versus non-linear ice sheet disintegration” and conclude:

The asymmetry of glacial-interglacial climate cycles, with rapid warming and sea level rise in the warming phase and a slower descent into ice ages, suggests that amplifying feedbacks can make the “wet” ice sheet disintegration process relatively rapid (Hansen et al., 2007). But how rapid? 

Paleoclimate records include cases in which sea level rose several meters per century, even though known natural positive forcings are much smaller than the human-made forcing. This implies that ice sheet disintegration can be a highly nonlinear process. 

We suggest that a nonlinear process spurred by an increasing forcing and amplifying feedbacks is better characterized by the doubling time for the rate of mass disintegration, rather than a linear rate of mass change. If the doubling time is as short as a decade, multi-meter sea level rise could occur this century. Observations of mass loss from Greenland and Antarctica are too brief for significant conclusions, but they are not inconsistent with a doubling time of a decade or less. The picture will become clearer as the measurement record lengthens. 

What constraints or negative feedbacks might limit nonlinear growth of ice sheet mass loss? An ice sheet sitting primarily on land above sea level, such as most of Greenland, may be limited by the speed at which it can deliver ice to the ocean via outlet glaciers. But much of the West Antarctic ice sheet, resting on bedrock below sea level, is not so constrained.

And so they end their paper with this prediction and warning:

IPCC BAU (business-as-usual) scenarios assume that greenhouse gas emissions will continue to increase, with the nations of the world burning most of the fossil fuels including unconventional fossil fuels such as tar sands.
An alternative extreme, one that places a substantial rising price on carbon emissions, would have CO2 emissions beginning to decrease within less than a decade, as the world moves on energy systems beyond fossil fuels, leaving most of the remaining coal and unconventional fossil fuels in the ground. In this extreme scenario, let’s call it fossil fuel phase-out (FFPO), CO2 would rise above 400 ppm but begin a long decline by mid-century (Hansen et al., 2008). 

The European Union 2 °C scenario, call it EU2C, falls in between these two extremes. 

BAU scenarios result in global warming of the order of 3-6 °C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain. Such a huge rapidly increasing climate forcing dwarfs anything in the paleoclimate record. Antarctic ice shelves would disappear, and the lower reaches of the Antarctic ice sheets would experience summer melt comparable to that on Greenland today. 

The other extreme scenario, FFPO, does not eliminate the possibility of multi-meter sea level rise, but it leaves the time scale for ice sheet disintegration very uncertain, possibly very long. If the time scale is several centuries, then it may be possible to avoid large sea level rise by decreasing emissions fast enough to cause atmospheric greenhouse gases to decline in amount. 

What about the intermediate scenario, EU2C? We have presented evidence in this paper that prior interglacial periods were less than 1 °C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters. It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal.

If Hansen and Sato are right, we will know within a decade or two.  Unfortunately, continuing to do nothing while we wait to find out all but ensures we cross the tipping point and enter the realm of worst-case scenarios.  Further delay is beyond immoral.

Link:  http://climateprogress.org/2011/01/20/hansen-sato-climate-tipping-point-multi-meter-sea-level-rise/#more-40818

"Paleoclimate Implications for Human-Made Climate Change" by James E. Hansen and Makiko Sato

Dr. Hansen has sent this draft out and welcomes suggestions:

Paleoclimate Implications for Human-Made Climate Change

by

James E. Hansen and Makiko Sato

NASA Goddard Institute for Space Studies and Columbia University Earth Institute, New York, NY, U.S.A.


ABSTRACT

Milankovic climate oscillations help define climate sensitivity and assess potential human-made climate effects.  We conclude that Earth in the warmest interglacial periods was less than 1 °C warmer than in the Holocene and that goals of limiting human-made warming to 2 °C and CO2 to 450 ppm are prescriptions for disaster. Polar warmth in prior interglacials and the Pliocene does not imply that a significant cushion remains between today's climate and dangerous warming, rather that Earth today is poised to experience strong amplifying polar feedbacks in response to moderate additional warming.  

Deglaciation, disintegration of ice sheets, is nonlinear, spurred by amplifying feedbacks.  If warming reaches a level that forces deglaciation, the rate of sea level rise will depend on the doubling time for ice sheet mass loss.  Gravity satellite data, although too brief to be conclusive, are consistent with a doubling time of 10 years or less, implying the possibility of multi-meter sea level rise this century.  The emerging shift to accelerating ice sheet mass loss supports our conclusion that Earth's temperature has returned to at least the Holocene maximum.  Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.

1.  Introduction

Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century.  The fate of humanity and nature may depend upon early recognition and understanding of human-made effects on Earth's climate (Hansen, 2009). Tools for assessing the expected climate effects of alternative levels of human-made changes of atmospheric composition include (1) Earth's paleoclimate history, showing how climate responded in the past to changes of boundary conditions including atmospheric composition, (2) modern observations of climate change, especially global satellite observations, coincident with rapidly changing human-made and natural climate forcings, and (3) climate models and theory, which aid interpretation of observations on all time scales and are useful for projecting future climate under alternative climate forcing scenarios.

This paper emphasizes information provided by paleoclimate data.  Milankovic climate oscillations, the glacial-interglacial climate swings associated with perturbations of Earth's orbit, provide a precise evaluation of equilibrium climate sensitivity, i.e., the response to changed boundary conditions after the atmosphere and ocean have sufficient time to restore planetary energy balance.  Implications become clearer when Pleistocene climate oscillations are viewed in the context of larger climate trends of the Cenozoic Era.  Ice cores and ocean cores are complementary tools for understanding, together providing a more quantitative assessment of the dangerous level of human interference with the atmosphere and climate.

Fig. 1 shows estimate global deep ocean temperature over the past 65.5 million years, the Cenozoic Era.  The deep ocean temperature is inferred from a global compilation of oxygen isotopic abundances in ocean sediment cores (Zachos et al., 2001), with the temperature estimate extracted from oxygen isotopes via the simple approximation of Hansen et al. (2008).

This deep ocean temperature change is similar to global surface temperature change, we

will argue, until the deep ocean temperature approaches the freezing point of ocean water.  Thus late Pleistocene glacial-interglacial deep ocean temperature changes (Fig. 1c) are only about two thirds as large as global mean surface temperature changes.

In this paper we discuss Cenozoic climate change and its relevance to understanding of human-made climate change.  We review how Milankovic climate oscillations provide a precise measure of climate sensitivity to any natural or human-made climate forcing.  We summarize how temperature is extracted from ocean cores to clarify the physical significance of this data record, because, we will argue, ocean core Milankovic data have profound implications about the dangerous level of human-made interference with global climate.  Finally we discuss the temporal response of the climate system to the human-made climate forcing.

2.  Cenozoic Climate Change

The Cenozoic era illustrates the huge magnitude of natural climate change.  Earth was so warm in the early Cenozoic that polar regions had tropical-like conditions – indeed, there were alligators in Alaska (Markwick, 1998).  There were no large ice sheets on the planet, so sea level was about 75 meters higher than today.

Earth has been in a long-term cooling trend for the past 50 million years (Fig. 1a). By approximately 34 Mya (million years ago) the planet had become cool enough for a large ice sheet to form on Antarctica. Ice and snow increased the albedo ('whiteness' or reflectivity) of that continent, an amplifying feedback that contributed to the sharp drop of global temperature at that time.  Moderate warming between 30 and 15 Mya was not sufficient to melt all Antarctic ice. The cooling trend resumed about 15 Mya and accelerated as the climate became cold enough for ice sheets to form in the Northern Hemisphere and provide their amplifying feedback.

The Cenozoic climate changes summarized in Fig. 1 contain insights and quantitative information relevant to assessment of human-made climate effects. Carbon dioxide (CO2) plays a central role in both the long-term climate trends and the short-term oscillations that were magnified as the planet became colder and the ice sheets larger.  Cenozoic climate change is discussed by Zachos et al. (2001), IPCC (2007), Hansen et al. (2008), and many others.  We describe here implications about the role of CO2 in climate change and climate sensitivity.

CO2 is the principal forcing that caused the slow Cenozoic climate trends over millions of 

years, as the solid Earth (volcanic) source altered the amount of CO2 in surface carbon reservoirs (atmosphere, ocean, soil and biosphere).  CO2 is also a principal factor in the short-term climate oscillations that are so apparent in parts (b) and (c) of Fig. 1.  However, in these glacial-interglacial oscillations atmospheric CO2 operates as a feedback: total CO2 in the surface reservoirs changes little on these shorter time scales, but the distribution of CO2 among the surface reservoirs changes as climate changes.  As the ocean warms, for example, it releases CO2 to the atmosphere, providing an amplifying climate feedback that causes further warming.

The fact that CO2 is the dominant cause of long-term Cenozoic climate trends is obvious from consideration of Earth's energy budget.  Such large climate changes cannot result from redistribution of energy within the climate system, as might be caused by changes of atmosphere or ocean dynamics.  Instead a substantial global climate forcing is required.  The climate forcing must be due to a change of energy coming into the planet or changes within the atmosphere or on the surface that alter the planet's energy budget.

Solar luminosity is increasing on long time scales, as our sun is at an early stage of solar evolution, "burning" hydrogen, forming helium by nuclear fusion, slowly getting brighter.  The sun's brightness increased steadily through the Cenozoic, by about 0.4 percent according to solar physics models (Sackmann et al., 1993).  Because Earth absorbs about 240 W/m² of solar energy, that brightness increase is a forcing of about 1 W/m²  This small linear increase of forcing, by itself, would have caused a modest global warming through the Cenozoic Era.

Read more here:  http://www.columbia.edu/~jeh1/mailings/2011/20110118_MilankovicPaper.pdf

Brad Johnson: The U.S. Energy Information Administration has projected that the United States will lead the world into catastrophic global warming over the next 25 years

EIA Projects Climate Catastrophe

by Brad Johnson, Wonk Room, Think Progress, December 21, 2010

The U.S. Energy Information Administration has projected that the United States will lead the world into catastrophic global warming over the next twenty five years. In its 2011 Annual Energy Outlook, the EIA predicts that energy-related CO2 emissions will “grow by 16% from 2009 to 2035,” reaching 6.3 billion metric tons of carbon dioxide equivalent (or 1.7 GtC):

The fuel mix the EIA projects remains predominantly coal and oil, with a moderate rise in renewable energy, whose pollution benefits are offset by growth in energy demand:

This pathway would almost certainly commit the world to catastrophic climate change, including rapid sea level rise, extreme famine, desertification, and ecological collapse on land and sea. Right now, the United States, with less than five percent of global population, produces 20% of global warming pollution. Center for American Progress senior fellow Joe Romm published in Nature in 2008 that humanity “must aim at achieving average annual carbon dioxide emissions of less than 5 GtC [5 billion metric tons of carbon, or 18 billion metric tons of carbon dioxide] this century or risk the catastrophe of reaching atmospheric concentrations of 1,000 p.p.m.” To do so, he said, humanity needs to adopt a “national and global strategy to stop building new traditional coal-fired plants while starting to deploy existing and near-term low-carbon technologies as fast as is humanly possible.”

Since 2008, the science has grown more dire. The impact of existing global warming on oceans, extreme weather, agriculture, polar ice, and ecosystems is at or exceeding the highest range of past projections. Dr. Romm’s suggestions were based on the assumption that stabilizing greenhouse gas concentrations at 450 ppm would likely limit warming to 2 °C above pre-industrial temperatures. However, as climate scientists Kevin Anderson and Alice Bows write in Philosophical Transactions of the Royal Society, “the impacts associated with 2 °C have been revised upwards, sufficiently so that 2 °C now more appropriately represents the threshold between ‘dangerous’ and ‘extremely dangerous’ climate change”:

There is now little to no chance of maintaining the rise in global mean surface temperature at below 2 °C, despite repeated high-level statements to the contrary. Moreover, the impacts associated with 2 °C have been revised upwards, sufficiently so that 2 °C now more appropriately represents the threshold between dangerous and extremely dangerous climate change.

Over a year and a half ago, Dr. Michael Mann concurred in Proceedings of the National Academy of Sciences that the 450 ppm target is “terribly risky“:

So regardless of one’s precise definition of dangerous anthropogenic interference, stabilizing greenhouse gas concentrations much above 450 ppm CO2eq would be a terribly risky prospect.

Friends of the Earth UK’s latest report, “Reckless Gamblers,” reflects the science in its recommendations for immediate and significant cuts in climate pollution, while admitting that there are significant global-scale risks that come even with that effort.

If future pollution is distributed on a per-capita basis, then net United States emissions would need to go to zero by 2030 (a similar effort by top climate institutes finds the US pathway goes to zero by 2020). Comparing the EIA pathway for energy-related CO2 emissions — which represent about 83% of total US greenhouse pollution — to the range of merely dangerous emissions pathways:

Unfortunately, the economics that policymakers rely upon is grossly outdated. Even as climate scientists have stopped considering 450 ppm stabilization safe, economists still question whether there would be any significant climate damage in a 550 ppm world (or even a 1000 ppm world). Economist Simon Dietz recently found that the risk of continent-scale economic disaster in a 550 ppm scenario is only 6% — and that’s dramatically higher than previous economic work. Based on his unreasonably sunny scenarios, he estimates that the “social cost of carbon” — essentially how current pollution should be taxed — is around $300/tCO2. And that’s dramatically higher than the official U.S. government estimates.

Suffice it to say our prospects for avoiding catastrophic loss caused by our damaged atmosphere are not improved by a political system in thrall to fossil fuel polluters. 

Hope for a sustainable future lies in our nation’s ability to overcome the fear of changing our disastrous status quo and conquer the great challenges ahead.

Link:  http://wonkroom.thinkprogress.org/2010/12/21/eia-climate-catastrophe/