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James Hansen on Climate Tipping Points and Political Leadership

In my opinion, it is still feasible to solve the global warming problem before we pass tipping points that would guarantee disastrous irreversible climate change. But urgent strong actions are needed.

It is clear that the required course is technically feasible, and it would have great benefits to the public in developing and developed countries. The geophysical facts practically dictate the way.

Unfortunately, knowledge and understanding of the situation are not widespread. In addition, there is a minority of people, termed “fossil interests,” who benefit from business-as-usual. These fossil interests have enormous influence on governments worldwide, far outside their fair role in democracies.

Climate Situation

Our global climate is nearing tipping points. Changes are beginning to appear, and there is a potential for rapid changes with effects that would be irreversible – if we do not promptly slow fossil fuel emissions during the next few decades.

Tipping points are fed by amplifying feedbacks. As Arctic sea ice melts, the darker ocean absorbs more sunlight and speeds melting. As tundra melts, methane a strong greenhouse gas, is released, causing more warming. As species are pressured and exterminated by shifting climate zones, ecosystems can collapse, destroying more species.

We already have caused atmospheric carbon dioxide to increase from 280 to 387 ppm (parts per million). What science has revealed in the past few years is that the safe level of carbon dioxide in the long run is no more than 350 ppm. The optimum CO2 level to support civilization may be less than 350 ppm, but more precise knowledge is not needed immediately for the purpose of establishing present policies.

The conclusion that CO2 must be reduced to a level below 350 ppm was startling at first, but obvious in retrospect.

Earth’s history shows that an atmospheric CO2 amount of say 450 ppm eventually would yield dramatic changes, including sea level tens of meters higher than today.

For reference, 450 ppm yields global warming about 2°C (3.6°F) above the preindustrial level. Such a level of atmospheric CO2 and global warming imply that we would hand our children and grandchildren a condition that would run out of their control, a situation that should be unacceptable to humanity.

Fossil Fuels

Human-made sources of atmospheric carbon dioxide are summarized in the graph at right. The dark portions of the bars are the portions of the fuels that have been burned with the carbon put into the atmosphere as carbon dioxide. The size of fossil fuel reserves (the fossil fuel not yet extracted from the ground) has significant uncertainty.

An important point is that the size of recoverable reserves depends upon whether drilling is allowed in off-shore regions, public lands, polar regions, and the deepest ocean. Similarly, the amount of coal reserves that is practically minable is uncertain and depends upon the degree to which ever more destructive mining practices are allowed. Unconventional fossil fuels (tar sands, oil shale, methane hydrates), not shown the first graph, are similar to coal in their high carbon content and have reserves that may be comparable in size to coal or even larger.

Despite uncertainties in reserve sizes, it is clear that if we burn all the fossil fuels, or even half of the remaining reserves, we will send the planet toward an ice-free state with sea level about 250 feet higher than today. It would take time for complete ice sheet disintegration to occur, but a chaotic situation would be created with changes occurring out of control of future generations.

Oil may already be about half depleted, i.e., the world may be close to peak oil production (implying that the IPCC estimate of reserves is closer to the truth than the EIA estimate). In either case, common sense suggests that the largest oil pools will be exploited and the carbon dioxide, which is emitted mainly from tailpipes, will end up in the atmosphere.

Gas, the least carbon intensive and cleanest burning fossil fuel, also surely will be exploited.

The obvious conclusion is that the only practical way to avoid climate catastrophe is to terminate emissions from the largest fossil fuel source: coal, the dirtiest of the fossil fuels. If coal emissions are phased out between 2010 and 2030, global fossil fuel emissions would begin to fall rapidly as shown in the chart below.

The rate of emissions (shown in billions of tons of carbon on the left and in percent of 2008 emissions on the right) depends upon how much oil and gas is used. The (red) curve showing larger emissions is based on the assumption that the larger reserve estimates of EIA are valid and all of the CO2 is emitted to the atmosphere. These emission scenarios have been converted to atmospheric CO2 amounts using a simplified version of the Bern carbon cycle model.

The next chart shows that atmospheric CO2 would peak at only ~400 ppm in ~2025, if the IPCC oil and gas reserves are accurate and if coal emissions are phased out uniformly over the period 2010-2030.

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