Assessing "Dangerous Climate Change": Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature

This paper, by an international team of scientists, points out the clear and present danger that today's children may be handed a deteriorating climate with consequences out of their control. 

Dr. James Hansen
by James Hansen, Pushker Kharecha, Makiko Sato, Valerie Masson-Delmotte, Frank Ackerman, David J. Beerling, Paul J. Hearty, Ove Hoegh-Guldberg, Shi-Ling Hsu, Camille Parmesan, Johan Rockstrom, Eelco J. Rohling, Jeffrey Sachs, Pete Smith, Konrad Steffen, Lise Van Susteren, Karina von Schuckmann, James C. Zachos

We conclude that the widely accepted target of limiting human-made global climate warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above the preindustrial level is too high and would subject young people, future generations and nature to irreparable harm. Carbon dioxide (CO2) emissions from fossil fuel use must be reduced rapidly to avoid irreversible consequences such as sea level rise large enough to inundate most coastal cities and extermination of many of today's species. Unabated global warming would also worsen climate extremes. In association with summer high pressure systems, warming causes stronger summer heat waves, more intense droughts, and wildfires that burn hotter. Yet because warming causes the atmosphere to hold more water vapor, which is the fuel that drives thunderstorms, tornadoes and tropical storms, it also leads to the possibility of stronger storms as well as heavier rainfall and floods. Observational data reveal that some climate extremes are already increasing in response to warming of several tenths of a degree in recent decades; these extremes would likely be much enhanced with warming of 2°C or more.

We use evidence from Earth's climate history and measurements of Earth's present energy imbalance as our principal tools for inferring climate sensitivity and the safe level of global warming. The inferred warming limit leads to a limit on cumulative fossil fuel emissions.

It is assessed that humanity must aim to keep global temperature close to the range occurring in the past 10,000 years, the Holocene epoch, a time of relatively stable climate and stable sea level during which civilization developed. The world cooled slowly over the last half of the Holocene, but warming of 0.8°C (1.4°F) in the past 100 years has brought global temperature back near the Holocene maximum.

We note that policies should emphasize fossil fuel carbon, not mixing in carbon from forest changes as if it were equivalent. Most of the carbon from fossil fuel burning will stay in the climate system for of order 100,000 years. Of course carbon dioxide from deforestation also causes warming and policies must address that carbon source, but good land use policies could restore most of that carbon to the biosphere on a time scale of decades to centuries. However, maximum biospheric restoration is likely to be only comparable to the past deforestation source, so fossil fuel sources must be strictly limited.

We conclude that human-made warming could be held to about 1°C (1.8°F) if cumulative industrial-era fossil fuel emissions are limited to 500 GtC (gigatons of carbon, where a gigaton is one billion metric tons) and if policies are pursued to restore 100 GtC into the biosphere, including the soil. This scenario leads to reduction of atmospheric CO2 to 350 ppm by 2100, as needed to restore Earth's energy balance and approximately stabilize climate.

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