Climate change

Key Concepts

• Climate change is happening now. While details vary from place to place, greenhouse gases from human activities—primarily the burning of fossil fuels such as coal and oil—are already causing global increases in air temperature, ocean heat content, and sea level; declines in glaciers, snow cover, and sea ice; and a rise in extreme weather-related events, such as tropical storms, floods, droughts, and forest fires.
• Climate change will continue for many decades to come. Even if greenhouse gas emissions stopped today, global temperatures would continue to rise and weather-related extremes would continue, as long-lasting greenhouse gases in the atmosphere continue to have their disruptive effects.
• Climate is a system. This means multiple climate elements—including rainfall, wind patterns, clouds, humidity, and soil moisture— affect each other in complex ways. Therefore, an increase in greenhouse gas emissions does not cause a linear, consistent increase in temperatures across the globe, but forces changes upon multiple climate-sensitive factors that will affect people and ecosystems around the world in varying ways.

A Few Facts to Know

• Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are the most abundant greenhouse gases produced by human activity. Although shorter lived in the atmosphere than carbon dioxide, methane is 20 times more powerful in producing warming in the atmosphere.
• There are natural variations in climate over time, but natural variation cannot account for the extremely rapid global warming trend currently underway. Human activity is key.
• In addition to a general increase in warming, climate change involves a change in the variability of climate factors, which translates to more weather-related extremes such as deadly heat waves, torrential precipitation, and droughts.
• Climate change is also causing changes in storm tracks, leading some storms to get “stuck” in one location and dump huge amounts of rain there.
• Scientists can now quantify with increasing precision the extent to which the frequency and intensity of extreme weather-related events is directly attributable to human-caused climate change.
• Increasing frequencies of heat waves and extremely heavy rains are among the shifts away from longstanding “normals” that can be most clearly attributed to climate change. By contrast, the links between climate change and the frequency of wildfires is relatively weak (although the links between climate change and size and intensity of wildfires—that is, once they start, how big they get) are relatively strong).
• There still are uncertainties about how ocean currents (major transporters of heat around the globe), cloud type and cover (which affects solar warming), and other dynamic elements of the climate system operate, and how they will respond to and contribute to climate disruption.

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