warming) and are clearly relevant to policy aimed at controlling emissions and reducing the risk of dangerous impacts. The approaches used here thereby provide additional policy-relevant information that would be lost in an analysis that only related impacts to CO2-equivalent concentration levels.


There are three key findings of this report, which correspond to the structure of this summary:

  1. Climate change in the very long term: Future stabilization targets correspond to altered states of Earth’s climate that would be nearly irreversible for many thousands of years, even long after anthropogenic greenhouse gas emissions ceased. The capacity to adapt to slow changes is generally greater than for near-term rapid climate change, but different stabilization levels can lock the Earth and many future generations of humans into large impacts that can occur very slowly over time, such as the melting of the polar ice sheets; similarly, some stabilization levels could prevent such changes.

  2. Climate change in the next few decades and centuries: Understanding the implications of future stabilization targets requires paying attention to the expected climate change and to the emissions required to achieve stabilization. Because of time lags inherent in Earth’s climate, the observed climate changes as greenhouse gas emissions increase reflect only about half of the eventual total warming that would occur for stabilization at the same concentrations. Moreover, emissions reductions larger than about 80% (relative to whatever peak global emission rate may be reached) are required to approximately stabilize carbon dioxide concentrations for a century or so at any chosen target level (e.g., 450 ppmv, 550 ppmv, 650 ppmv, 750 ppmv, etc.).4 Even greater reductions in emissions would be required to maintain stabilized concentrations in the longer term. It should be emphasized that this finding is not linked to any particular policy choice about time of stabilization or stabilization concentration, but applies broadly, and is due to the fundamental physics of the carbon cycle presented in Chapter 2.

  3. Climate changes, impacts, and choices among stabilization targets: A number of key climate changes and impacts for the next few decades and


In this report the mixing ratio for any compound, CO2 for example, is expressed in either ppm (parts per million, i.e., the number of molecules of CO2 for every million molecules of air) or in ppmv (parts per million volume, the ratio of CO2 to air calculated in volumes) but used equivalently.

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