Our Changing Climate: National Climate Assessment NCA4 Vol. 2, Chapter 2

Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout history. Global average temperature has increased by about 1.7°F from 1901 to 2016. Observational evidence does not support any credible natural explanations for this amount of warming. The evidence consistently points to the emissions of greenhouse gases by humans as the dominant cause.

How much more the climate changes will depend primarily on global emissions of greenhouse gases and on the response of the climate system to human-induced warming. With significant emission reductions, global temperature increase could be limited to 3.6°F or less. Without significant reductions, global temperatures could increase by 9°F or more by the end of this century.

Global average sea level has risen by about 7-8 inches since 1900, with almost half this rise occurring since 1993 as oceans have warmed and land-based ice has melted. Sea levels are very likely to continue to rise by at least 1-4 feet by 2100 relative to 2000 levels. Emerging science regarding Antarctic ice sheet stability suggests that for higher scenarios a rise exceeding 8 feet is physically possible.

In the Arctic, average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost, loss of sea ice and glacier mass. By mid-century, it is very likely that the Arctic will nearly free of sea ice in late summer. Permafrost is expected to continue to thaw and methane released from thawing permafrost has potential to amplify human-induced warming.

Human-induced change is contributing to the poleward expansion of the tropics and a northward shift in Northern Hemisphere winter storm tracks since 1950. Increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity. Hurricane rainfall and intensity are projected to increase, as are the frequency and severity of landfalling atmospheric rivers on the West Coast.

Climate change resulting from human activities will persist for millennia. Future changes above the range projected by climate models cannot be ruled out and due to their systematic tendency to underestimate temperature change during past warm periods, models may be more likely to underestimate than to overestimate long-term future change.