We Have a Decade

James Hansen’s speech (pdf) at the December meeting of the American Geophysical Union implies that there is a much better sense of what is going to happen with the climate, and that the changes will be significantly faster than was expected.

Note: some temperature increases refer to since 1900, others are on top of the current increase (now at 0.8 C).

[T]o find a planet 2 or 3°C warmer than now, as it will be this century in “business-as-usual” scenarios, we must go back to the middle Pliocene, about 3 million years ago. At that time sea level was 25 ± 10 m greater than today…

A key question: what is the ice sheet response time? If it is centuries, and if we have business-as-usual CO2 growth, then, since thermal expansion of ocean water and alpine glaciers would be expected to add half a meter of sea level, total sea level rise might be a couple of meters this century, and several more next century.

If climate warms 2-3°C, and ice sheets begin to adjust to an equilibrium size consistent with that climate, a time scale of centuries would offer no consolation to coastal dwellers, because they would be faced with irregular incursions associated with storms and thus needing to continually rebuild above a transient water level….

Global warming also increases snowfall, so the interiors of Greenland and Antarctica are getting thicker. Because of this competing effect, sea level changes slowly at first, but as global warming gets larger, as summer melt extends higher up the ice sheet, and as buttressing ice shelves melt away, multiple positive feedbacks come into play, and the nonlinear disintegration wins the competition hands down…

If we follow a business-as usual scenario, we will be creating a hammer hitting the Earth faster and harder than it has ever been hit. Except perhaps when the Earth was hit by the asteroid that killed the dinosaurs.

The equilibrium sea level response to 3°C global warming is about 80 feet, based on the history of the Earth. The fact that the ice sheet response time is probably hundreds of years is no solace to coastal dwellers, because they will be faced with irregular incursions associated with storms, and will need to continually rebuild above a transiently rising sea level….

Showing a map of the world, how much will the change in temperature from 1900 compare to the typical changes seen year to year? How much will summer temperatures rise? In one scenario examined, most of Eurasia and Africa, and about half of the US, show a temperature increase of 4 to 12 C, 10 – 53 times the standard deviation of observed changes between 1900 and 2000. In the second scenario, temperature increases in the same region are 3 – 4 C, about 5 – 10 times the standard deviation, etc. In the third, alternative, scenario, with rapid attention to climate change, the same regions show a temperature increase of some 1 – 2 C or less, about 2 – 10 times the standard deviation, etc.

On the right is the ratio of the mean warming to the local standard deviation of seasonal mean temperature in the 20th century. The standard deviation includes the effect of both year-to-year variations and long-term trend. I submit that changes in the mean by 5-10 standard deviations in the business-as-usual scenarios are prima facie evidence of dangerous human-made interference. Changes of 5 to 10 standard deviations mean that the environment and its inhabitants would be facing average local conditions that they had not experienced before even in the most extreme years….

In summary, with regard to regional climate: as with global climate and sea level, business-as- usual scenarios will produce basically another planet. How else can you describe climate change in which the Arctic becomes an open lake in the summer and fall, and most land areas on Earth experience mean warming this century that is 5-10 times larger than the standard deviation of the past century?…

So, in summary, is there still time to avoid dangerous human-made interference with climate? I believe the evidence shows with reasonable clarity that the level of additional global warming that would put us into dangerous territory is about 1°C, not 2 or 3°C. We will need to refine our estimate as more data comes in, but I am quite confident of this assertion.

Yes, it is technically possible to avoid the grim “business-as usual” climate change, to follow an alternative scenario in which growth of greenhouse gas emissions is slowed in the first quarter of this century, primarily via concerted improvements in energy efficiency and a parallel reduction of non-CO2 climate forcings, and then reduced via advanced energy technologies that yield a cleaner atmosphere as well as a stable climate. The required actions make practical sense and have other benefits, but they will not happen without strong policy leadership and international cooperation. Action must be prompt, otherwise CO2-producing infrastructure that may be built within a decade will make it impractical to keep further global warming under 1°C.

I refer especially to the large number of coal-fired power plants that China, the United States, and India are planning to build without CO2 sequestration.

I said that I would return to the question of why, if an alternative scenario is practical, has multiple benefits, and makes good common sense, why are we not doing it?

There is little merit in casting blame for inaction, unless it helps point toward a solution. It seems to me that special interests have been a roadblock wielding undue influence over policymakers. The special interests seek to maintain short-term profits with little regard to either the long-term impact on the planet that will be inherited by our children and grandchildren or the long-term economic well-being of our country.

The public, if well-informed, has the ability to override the influence of special interests, and the public has shown that they feel a stewardship toward the Earth and all of its inhabitants.

What is the alternative behavior which can get us to the alternative scenario?

On the long run, satisfying energy needs while decreasing CO2 emissions will require development of renewable energies, sequestration of CO2 produced at power plants, and perhaps a new generation of nuclear power. But a flattening out of emissions can be achieved now with improved energy efficiency. It is important that the United States, as a technology leader and as the largest producer of CO2 in the world, take a leadership role.

How much do we need to cut carbon emissions?

If we wanted to stabilize atmospheric CO2 for the next few years, we would need to cut fossil fuel emissions by about 60% [as the oceans and reforestation can absorb 40% of what we absorb, at least for the short term]. But, because of backpressure from CO2 added to the ocean, in the long run the cut in emissions must be larger than 60% to stabilize atmospheric amount.

Next post: how rapidly do we need to cut carbon emissions?

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