Anticipated impacts. The graphical part of the diagram shows how atmospheric concentrations of carbon dioxide correspond to temperatures—transient, or near-term warming (in blue), is only a fraction of the total warming—the equilibrium warming—expected to occur (in red). Click to enlarge.

The report stresses that choices made now about carbon dioxide emissions reductions will affect climate change impacts experienced not just over the next few decades but also in coming centuries and millennia. Because CO₂ in the atmosphere is long-lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe.

However, the report does not recommend any particular stabilization target, noting that choosing among different targets is a policy choice rather than strictly a scientific one because of questions of values regarding how much risk or damage to people or to nature might be considered too much.

Although some important future effects of climate change are difficult to quantify, there is now increased confidence in how global warming of various levels would relate to several key impacts, says the report. It lists some of these impacts per degree Celsius (or per 1.8 ° Fahrenheit) of global warming, for example (these apply for 1 °C to 4 °C of warming):

• 5% to 10% less total rain in southwest North America, the Mediterranean, and southern Africa per degree Celsius of warming.
• 5% to 10% less streamflow in some river basins, including the Arkansas and Rio Grande, per degree Celsius of warming.
• 5% to 15% lower yields of some crops, including US and African corn and Indian wheat, per degree Celsius of warming.

While total rain is expected to decrease in some areas, more of the rain that does occur is expected to occur in heavy falls in most land areas (3% to 10% more heavy rain per degree Celsius). In addition, warming of 1 °C to 2 °C (1.8 °F to 3.6 °F) could be expected to lead to a two-fold to four-fold increase per degree in the area burned by wildfire in parts of western North America, the report says.

Warming of 3 °C (5.4 °F) would put many millions more people at risk of coastal flooding and lead to the loss of about 250,000 square km of wetlands and drylands.

Warming of 4 °C (7.2 °F) would lead to far warmer summers; about nine out of 10 summers would be warmer than the warmest ever experienced during the last decades of the 20^th century over nearly all land areas.

Currently the concentration of CO₂ in the atmosphere is about 390 parts per million volume (ppmv), the highest level in at least 800,000 years. Depending on emissions rates, that level could double or nearly triple by the end of the century, greatly amplifying future human impacts on climate, the report says.

Because the amount of human-caused CO₂ emissions already far exceeds the amount that can be removed through natural carbon sinks such as oceans, keeping emissions rates the same will not stabilize the concentration of CO₂ in the atmosphere. Even if emissions held steady, the CO₂ concentration in the atmosphere would increase, much like the water level in a bathtub when water is coming in faster than it is draining. Emissions reductions larger than about 80%, relative to whatever peak global emissions rate may be reached, would be required to approximately stabilize carbon dioxide concentrations for a century or so at any chosen target level.

Further, stabilizing atmospheric concentrations does not mean that temperatures will stabilize immediately. Warming that occurs in response to a given increase in the CO₂ concentration is only about half the total warming that will ultimately occur. For example, if the CO₂ concentration stabilizes at 550 ppmv, the Earth would warm about 1.6 °C on the way to that level; but even after the CO₂ level stabilizes, the warming would continue to grow in the following decades and centuries, reaching a best-estimate global equilibrium warming of about 3 °C (5.4 °F). Waiting to observe impacts before choosing a stabilization target would therefore imply a lock-in to about twice as much eventual crop loss, rainfall changes, and other impacts that increase with warming.

The report offers likely ranges and best estimates of the equilibrium warming that can be expected from various levels of CO₂ in the atmosphere.

The report was sponsored by the Energy Foundation and the US Environmental Protection Agency. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies.

Resources

• Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia