The role of coal in global warming is clarified by a small number of well-documented facts. Figure 1 shows the fraction of fossil fuel carbon dioxide (CO2) emissions that remains in the air over time. One-third of the CO2 is still in the air after 100 years, and one-fifth is still in the air after 1000 years.
Figure 1. The fraction of CO2 remaining in the air, after emission by fossil fuel burning, declines rapidly at first, but 1/3 remains in the air after a century and 1/5 after a millennium 
Oil slightly exceeds coal as a source of CO2 emissions today, as shown in Figure 2a. [IPCC = Intergovernmental Panel on Climate Change; WEC = World Energy Council] But, because of the long atmospheric lifetime of past emissions, fully half of the excess CO2 in the air today (from fossil fuels), relative to pre-industrial times, is from coal (Figure 2b). Moreover, coal use is now increasing, while oil production has stagnated. Oil production will peak and will be constrained by available resources earlier than will coal production.
Figure 2. Percent contributions of different fossil fuels to 2006 CO2 emissions (left side) and contributions to the excess CO2 in the air today relative to pre-industrial CO2 amount 
Figure 3. Estimated fossil fuel reserves; purple portions have already been used 
Figure 3 shows reported fossil fuel reserves and resources (estimated undiscovered deposits). Reserves are hotly debated and may be exaggerated, but we know that enough oil and gas remain to take global warming close to, if not into, the realm of dangerous climate effects. Coal and unconventional fossil fuels such as tar shale contain enough carbon to produce a vastly different planet, a more dangerous and desolate planet, from the one on which civilization developed, a planet without Arctic sea ice, with crumbling ice sheets that ensure sea level catastrophes for our children and grandchildren, with shifting climate zones that cause great hardship for the world’s poor and drive countless species to extinction, and with intensified hydrologic extremes that cause increased drought and wildfires but also stronger rain, floods, and storms.
Oil and coal uses differ fundamentally. Oil is burned primarily in small sources, in vehicles where it is impractical to capture the CO2 emissions. Available oil reserves will be exploited eventually, regardless of efficiency standards on vehicles, and the CO2 will be emitted to the atmosphere. The climate effect of oil is nearly independent of how fast we burn the oil, because much of the CO2 remains in the air for centuries. [It is nevertheless important to improve efficiency of oil use, because that buys us time to develop technologies and fuels for the post-oil era, and high efficiency surely will be needed in the post-oil era.] However, the point is this: oil will not determine future climate change. Coal will.
Avoiding dangerous atmospheric CO2 levels requires curtailment of CO2 emissions from coal. Atmospheric CO2 can be stabilized by phasing out coal use except where the CO2 is captured and sequestered, as is feasible at power plants. Indeed, agreement to phase out coal use except where the CO2 is captured is 80% of the solution to the global warming crisis. Of course, it is a tall order, as coal is now the world’s largest source of electrical energy. Over the next few decades those coal plants must be closed or made to capture their CO2 emissions. Yet it is a doable task. Compare that task, for example, with the efforts and sacrifices that went into World War II.
Responsibility for Global Warming
Responsibility for global warming is proportional to cumulative CO2 emissions, not to current emission rates . This is physical fact, not an ethical statement. It is a consequence of the long lifetime of atmospheric CO2. Responsibility of the United States is more than three times larger than that of any other nation (Figure 4). Despite rapid growth of emissions from China, the United States will continue to be the nation most responsible for climate change for at least the next few decades.
Figure 4. Annual and cumulative fossil fuel CO2 emissions by country of emission 
It is also useful to examine per capita fossil fuel CO2 emissions. Figure 5a shows per capita emissions for the eight nations with largest total emissions, in order of decreasing total emissions. The United States and Canada have the largest per capita emissions, while emissions of Japan, Germany and the United Kingdom are half as large per capita.
Figure 5. Per capita fossil fuel emissions (a) in order of national emissions today, (b) per capita cumulative emission (2006 population) in order of national cumulative emissions 
Per capita responsibility for climate change, however, must be based on cumulative national emissions. The United Kingdom has the highest cumulative emissions per capita (2006 population), as shown in Figure 5b. The United States is second in per capita emissions and Germany is third. Increased per capita responsibility of the United Kingdom and Germany is a consequence of their early entries into the industrial era. Recognition of these facts is not an attempt to cast blame. Early emissions of CO2 occurred before the climate problem was recognized and well before it was proven. Yet these facts are worth bearing in mind.
Human-made climate change is unambiguously underway. Yet the urgency of the situation is not readily apparent to everybody. Chaotic weather fluctuations mask climate trends, even as climate change alters the nature of weather. Urgency is created by the very inertia of the climate system that delays the effects of gases already added to the air.
This delay means that there is additional global warming “in the pipeline” due to human-produced gases already in the air. Climate system inertia is due in part to the massive oceans, four kilometers deep on average, which are slow to warm in response to increasing greenhouse gases. The effect of this inertia is compounded by positive (amplifying) feedbacks, such as melting of ice and snow, which increases absorption of sunlight, engendering more melting. Such feedbacks are not “runaway” processes, but they make climate sensitive to even moderate climate forcings. [A climate forcing, natural or human-made, is an imposed perturbation of the planet’s energy balance. Examples include a change of the sun’s brightness or an increase of long-lived greenhouse gases, which trap the Earth’s heat radiation.]
Climate inertia and positive feedbacks together create the danger of passing climate “tipping points”. A tipping point exists when the climate reaches a point such that no additional forcing is needed to instigate large, relatively rapid climate change and impacts. Impacts of these large climate changes tend to be, overall, detrimental to humans, because civilization is adapted to the relatively stable interglacial period that has existed on our planet for about ten thousand years, and we have settled the land and built great infrastructure within and upon these relatively stable climate zones and coastlines.
Based on current information, we now realize that we have passed or are on the verge of passing several tipping points that pose grave risks for humanity and especially for a large fraction of our fellow species on the planet. This information is gleaned primarily from the Earth’s history and ongoing global observations of rapid climate changes, and to a lesser extent from climate models that help us interpret observed changes.
Potential consequences of passing these tipping points include:
(1) loss of warm season sea ice in the Arctic and thus increased stress on many polar species, possibly leading to extinctions,
(2) increasing rates of disintegration of the West Antarctic and Greenland ice sheets, and thus more rapidly rising sea levels in coming decades,
(3) expansion of sub-tropical climates adversely affecting water availability and human livability in regions such as the American West, the Mediterranean, and large areas in Africa and Australia,
(4) reduction of alpine snowpack and water run-off that provides fresh water supplies for hundreds of millions of people in many regions around the world, and
(5) increased intensity of the extremes of the hydrologic cycle, including more intense droughts and forest fires, on the one hand, but also heavier rains and greater floods, as well as stronger storms driven by latent heat, including tropical storms, tornados and thunderstorms.
The nearness of these climate tipping points is no cause for despair. On the contrary, the actions that are needed to avert the tipping point problems are not only feasible, they have side benefits that point to a brighter future for life on the planet, with cleaner air and cleaner water. It will be necessary to roll back the airborne amounts of several air pollutants, but that is plausible, given appropriate attention. Already all pollutants except CO2 are falling at or below the lowest IPCC (Intergovernmental Panel on Climate Change) scenarios, and there is much potential for further reductions.
The tendency of the media to continually report bad news on climate change and the human-made factors that drive climate change sometimes paints a picture that is bleaker than that shown by careful analysis. Such information is often misleading about the true status of the Earth, and the impression created may be harmful if it leads to despair about the prospects for achieving a relatively stable climate with a cleaner atmosphere and ocean. I illustrate with data for CO2, the most important climate forcing.
Figure 6 is the “airborne.fraction” of fossil fuel CO2 emissions. This is the ratio: the annual increase of CO2 that appears in the Earth’s atmosphere (well measured) divided by the annual human emission of fossil fuel CO2 into the air (also well known). On average, the increase of CO2 in the air is 57% of the fossil fuel emissions. Although this is a large amount, the 43% taken up by the ocean, soil and biosphere is also large. The uptake is large despite the fact that humans are also causing extensive, mostly unwise, deforestation, which adds CO2 to the air. In addition our agricultural practices typically do not encourage storage of carbon in the soil.
Figure 6. Ratio of observed atmospheric CO2 increase to fossil fuel CO2 emissions 
There is tremendous potential for reducing atmospheric CO2 via reduction of deforestation, improved forestry practices, and improved agricultural practices that increase carbon storage in the soil. If governments were to encourage such practices, rather than the converse, and if coal use were phased out except where the CO2 is captured, it would be possible to literally roll back the net human-made climate forcing to levels below those defining critical tipping points.
We must remember, at the same time, that the ability of the principal CO2 sink, the ocean, to soak up human-made emissions is limited and slow (Figure 1). If we burn most of the available coal (Figure 3) without CO2 capture, even with the lowest estimates of available coal reserves, it will be impractical if not impossible to avoid passing climate tipping points with disastrous consequences.
Summary: The Need for Leadership
I am optimistic that greenhouse gas emissions can be reduced and atmospheric composition stabilized at a level avoiding disastrous climate effects. My optimism is based in part on the fact that young people are beginning to make their voices heard. They have a powerful effect on our consciences, with an ability to influence policy makers and the captains of industry.
Many individuals are beginning to recognize and appreciate the nature of the climate problem. People want to take actions and they are willing to make sacrifices. However, individual actions cannot solve the problem by themselves.
Based on fossil fuel and carbon cycle facts summarized above, we cannot continue to burn the coal reserves without CO2 capture and sequestration. Solution of this problem can be achieved only via strong government leadership.
Governments must recognize the relative magnitudes of fossil fuel resources, i.e., oil, gas, coal, and unconventional fossil fuels, and they must establish policies that influence consumption in ways consistent with preservation of our climate and life on Earth. The fossil fuel facts dictate essential actions :
(1) Phase-out of coal use that does not capture CO2. This is 80% of the solution, creating a situation in which CO2 emissions are declining sharply. (Coal use will also be affected by the second essential action. Indeed, it is likely that much of the coal will be left in the ground, as economic incentives spark innovations and positive feedbacks, accelerating progress to the cleaner world beyond fossil fuels.)
(2) A gradually but continually rising price on carbon emissions. This will ensure that, as oil production inevitably declines, humanity does not behave as a desperate addict, seeking every last drop of oil in the most extreme pristine environments and squeezing oil from tar shale, coal, and other high-carbon sources that would ensure destruction of our climate and most species on the planet. Recognition by industry of a continually rising carbon price (and elimination of fossil fuel subsidies) would drive innovations in energy efficiency, renewable energies, and other energy sources that do not produce greenhouse gases.
These are the two fundamental actions that must occur if we are to roll back the net climate forcing and avoid the dangerous climate tipping points, with their foreseeable consequences. Both of these actions are essential.
We can make a long list of supplementary actions that will be needed to avoid hardships and minimize dislocations as we phase into a cleaner world beyond fossil fuels. However, the two essential actions must be given priority and governments must explain the situation to the public.
Supplementary actions include improved efficiency standards on buildings, vehicles, appliances, etc. Rules must be changed so that utilities profit by encouraging efficiency, rather than selling more energy. These changes are necessary for success, and there are many economic opportunities associated with them. Yet governments must realize the essential actions dictated by the physics of the carbon cycle. Specifically, release of CO2 to the air from the large carbon reservoirs, coal and unconventional fossil fuels, must be curtailed.
Further actions will be needed to achieve a rollback of the net climate forcing. These actions  include reduction of non-CO2 climate forcings and improved agricultural and forestry practices. These actions are important and have multiple benefits, especially in developing countries, but they do not have the great urgency of halting construction of new coal plants without carbon capture. Power plants have long lifetimes, and once their CO2 is released to the air, it is impractical to recover it.
Energy departments, influenced by fossil fuel interests, take it as a God-given fact that we will extract all fossil fuels from the ground and burn them before we move on to other ways of producing usable energy. The public is capable of changing this course dictated by fossil fuel interests, but clear-sighted leadership is needed now if the actions are to be achieved in time.
Tipping points and positive feedbacks exist among people, as well as in the climate system. I believe that the action with the greatest potential to initiate positive feedbacks, and lead to the benefits that will accompany a clean energy future, is a moratorium in the West on new coal-fired power plants unless and until CO2 capture and sequestration technology is available. Such a moratorium would provide the West with sufficient moral authority to sit down with China and other developing countries to find ways, likely including technological assistance, for developing countries to also phase out coal use that does not capture CO2.Perhaps the most important question is this: can we find the leadership to initiate the tipping point among nations? Can we find a country that will place a moratorium on any new coal-fired power plants unless they capture and store the CO2? Unless this happens soon, there is little hope of avoiding the climate tipping points, with all that implies for life on this planet.
 Reprinted from a publically circulated letter by James Hansen as a private citizen to Chancellor Angela Merkel, Federal Republic of Germany dated 22 January 2008, with kind permission from James Hansen. Prof. Hansen is the director of the NASA Goddard Institute for Space Studies and Adjunct Professor at the Columbia University Earth Institute. The use of man made carbonates to sequester atmospheric CO2 as advocated by TecEco is not mentioned.
 Atmos. Chem. Phys. 7, 2287-2312, 2007
 CDIAC data for 1751-2004, BP for 2005-6; cf. Atmos. Chem. Phys. 7, 2287-2312, 2007
Proc. Natl. Acad. Sci. 101, 16109-16114, 2004