KEY FINDINGS

1. Climate change in the very long term: Future stabilization targets correspond to altered states of the 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 the 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, emission 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.), 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 centuries can now be identified and estimated at different levels of warming. Many impacts can be shown to scale with warming (see Figure S.5). Scientific progress has resulted in increased confidence in the understanding how global warming levels of 2, 3, 4, 5°C, etc. (see Figure S.1) affect precipitation patterns, extreme hot seasons, streamflow, sea ice retreat, reduced crop yields, coral bleaching, and sea level rise. This increased confidence provides direct scientific support for evaluating the implications of different stabilization targets. However, other climate changes and impacts are currently understood only in a qualitative manner. Many potential effects on human societies and the natural environment cannot presently be quantified as a function of stabilization target (see Figure S.6). This shortcoming does not imply that these changes and impacts are negligible. Some of these impacts, such as species changing their ranges or behavior, could be very important; indeed, some may dominate future risks due to anthropogenic climate change. Uncertainty in the carbon dioxide emissions and concentrations corresponding to a given temperature target is large, and choices about stabilization targets depend upon judgments regarding the degree of acceptable risk associated with both quantifiable and non-quantifiable impacts and changes.

Taken from NAS