How Can We Avert Dangerous Climate Change?

Reprint from James Hansen; dieser Artikel wird noch ins Deutsche übersetzt.

Recent analyses indicate that the amount of atmospheric CO2 required to cause dangerous climate change is at most 450 ppm, and likely less than that. Reductions of non-CO2 climate forcings can provide only moderate, albeit important, adjustments to the CO2 limit. Realization of how close the planet is to ‘tipping points’ with unacceptable consequences, especially ice sheet disintegration with sea level rise out of humanity’s control, has a bright side.

It implies an imperative: we must find a way to keep the CO2 amount so low that it will also avert other detrimental effects that had begun to seem inevitable, e.g., ocean acidification, loss of most alpine glaciers and thus the water supply for millions of people, and shifting of climatic zones with consequent extermination of species.

Here I outline from a scientific perspective actions needed to achieve low limits on CO2 and global warming. These changes are technically feasible and have ancillary benefits. Achievement of needed changes requires overcoming the spurious argument that developed and developing countries have equivalent responsibilities, as well as overcoming special interests advocating minimalist or counterproductive actions such as corn-based ethanol and liquid-fuel-from-coal programs.

This paper consists of written testimony that I delivered as a private citizen to the Select Committee on Energy Independence and Global Warming, United States House of Representatives on 26 April 2007. I have added to that testimony: this abstract, references for several statements in the testimony, and some specificity in the final section on solutions.

1. Summary

Crystallizing scientific data and analyses reveal that the Earth is close to dangerous climate change, to tipping points of the system with potential for irreversible deleterious effects. This information derives in part from paleoclimate data, i.e., the record of how climate changed in the past, as well as from measurements being made now by satellites and in the field.

The Earth’s history shows that climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This has allowed the entire planet to be whipsawed between climate states. Huge natural climate changes, from glacial to interglacial states, have been driven by very weak, very slow forcings, and positive feedbacks.

Now humans are applying a far stronger forcing much more rapidly, as we put back into the atmosphere, in a geologic heartbeat, fossil fuels that accumulated over millions of years. Positive feedbacks are beginning to occur, on a range of time scales.

The climate system has inertia. Nearly full response to a climate forcing requires decades to centuries. But that inertia is not our friend. It means that there is additional climate change in the pipeline that will occur in coming decades even without additional greenhouse gases.

The upshot is that very little additional forcing is needed to cause dramatic effects. To cause the loss of all summer Arctic ice with devastating effects on wildlife and indigenous people. To cause an intensification of subtropical conditions that would greatly exacerbate water shortages in the American West and many other parts of the world, and likely render the semi-arid states from west and central Texas through Oklahoma, Kansas, Nebraska and the Dakotas increasingly drought prone and unsuitable for agriculture. To cause the extermination of a large fraction of plant and animal species, an indictment of humanity’s failure to preserve creation.

For humanity itself, the greatest threat is the likely demise of the West Antarctic ice sheet as it is attacked from below by a warming ocean and above by increased surface melt. There is increasing realization that sea level rise this century may be measured in meters if we follow business-as-usual fossil fuel emissions.

There is a bright side to this planetary emergency. We can successfully address the emergency only by stabilizing climate close to its present state; there is no viable option, as adaptation to a continually rising sea level is not practical. Therefore, if we address the problem, there will be no need to adapt to the highly deleterious regional climate changes mentioned above, acidification of the ocean, and other detrimental effects. The actions needed to stabilize climate will preserve creation and restore a cleaner, healthier atmosphere.

The dangerous level of CO2 is at most 450 ppm, and it is probably less. The low limit on CO2 forces us to move promptly to the next phase of the industrial revolution. Changing light bulbs and making ethanol from corn will not solve the problem, although the former act is useful. Science provides a clear outline for what must be done, a four point strategy:

First, we must phase out the use of coal and unconventional fossil fuels except where the CO2 is captured and sequestered. There should be a moratorium on construction of old-technology coal-fired power plants.

Second, there must be a rising price (tax) on carbon emissions, as well as effective energy efficiency standards, and removal of barriers to efficiency. These actions are needed to spur innovation in energy efficiency and renewable energies, and thus to stretch oil and gas supplies to cover the need for mobile fuels during the transition to the next phase of the industrial revolution ‘beyond petroleum’.

Third, there should be focused efforts to reduce non-CO2 human-made climate forcings, especially methane, ozone and black carbon.

Fourth, steps must be taken to ‘draw down’ atmospheric CO2 via improved farming and forestry practices, including burning of biofuels in power plants with CO2 sequestration.

Note that I do not specify an exact fraction by which CO2 emissions must be reduced by 2050 or any other date. Indeed, science is not able to specify an exact requirement now, but we can say that emissions must be reduced to a fraction of their current values. Given the fact that readily available oil will surely be employed for mobile sources, and given the magnitudes of the different fossil fuel reservoirs, it seems best to frame the problem as I have in this four-point strategy, and adjust specific targets and policies as knowledge improves.

Responsibility of the United States for global climate change exceeds that of any other nation by more than a factor of three, even though China is now passing the United States in current emissions. The United States will continue to be primarily responsible for climate change for decades to come.

The above conclusions follow from the science. I go further here in expressing my opinion about the implications of this research for citizens in our democratic system. I believe that this is appropriate, in part because of resistance that the scientific conclusions have met among special interests, and because of misinformation about the science that has been spread. My opinions carry no more weight than those of any other citizen, but conceivably my experience in presenting this research in different circles allows some insight. In any case, I have as much right to express my opinion as do the special interests.

In my opinion, the United States should recognize openly its leading role in causing human-made climate change and promptly take a leadership role in addressing the matter. We have a moral responsibility to do so.

Moreover, it is in our interest to take actions now. We can benefit economically from extensive technology development, with many good high-tech high-pay jobs. Of course, moving to the next phase of the industrial revolution will require changes, dislocations, sacrifices and hard work. But these provide no basis for inaction.

We cannot let the pleadings and misinformation of special interests determine our actions, special interests driven by motives of short-term profit. And we cannot shrink from our personal responsibilities. We are now, through our government, standing alongside the polluters, officially as a hulking ‘friend of the court’, arguing against limitations on emissions.

Is this the picture of our generation we will leave for our children, a picture of ignorance and greed? We live in a democracy. Policies represent our collective will. We cannot blame others. If we allow the planet to pass tipping points, to set in motion irreversible changes to the detriment of nature and humanity, it will be hard to explain our role to our children and grandchildren.

We cannot claim, with legitimacy, that ‘we did not know’. In my opinion, it is time for the public to demand, from government.

To read the full paper, please go to

Maiken Winter

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