Warming directly makes heat waves longer, stronger, and more frequent. For instance, a major 2012 study found that extreme heat waves in Texas, such as the one that occurred in 2011, are much more likely—20 times more likely in years like 2011—to occur than they were 40–50 years ago.
Although human-caused global warming makes extremely warm days more likely, it makes extremely cold days less likely. So while we will continue to have record-setting cold temperatures in places, the ratio of record-setting hot days to record-setting cold days will grow over time, which has been measured. The U.S. National Center for Atmospheric Research (NCAR) reported in late 2009 that “Spurred by a warming climate, daily record high temperatures occurred twice as often as record lows over the last decade across the continental United States, new research shows.” Likewise, the UK Met Office reported in 2014 that, globally, the ratio of days that are extremely warm versus the days that are extremely cold has risen sharply since 1950. They point out “Globally, 2013 was also in the top 10 years for the number of warm days and in the bottom 10 years for the number of cool nights since records began in 1950.”
Global warming directly makes droughts more intense by drying out and heating up land that is suffering from reduced precipitation. The warming also worsens droughts by causing earlier snowmelt, thus reducing a crucial reservoir used in the West during the dry summer season. Finally, climate change shifts precipitation patterns, causing semi-arid regions to become parched. For instance, the 2012 Texas study found “indications of an increase in frequency of low seasonal precipitation totals.”
The heat and the drying and the early snow melt also drive worsening wildfires, particularly in the West. The wildfire season is already more than 2 months longer than it was just a few decades ago, and wildfires are much larger and more destructive.
Warming also puts more water vapor in the atmosphere, so that wet areas of the world become wetter and deluges become more intense and more frequent. This effect has already been documented and linked to human activity in the northern hemisphere. As New York Governor Andrew Cuomo said after Superstorm Sandy slammed his state just 2 years after it was deluged by hurricane Irene, “We have a one-hundred year flood every two years now.” Note that this means that when it is cold enough to snow, snowstorms will be fueled by more water vapor and thus be more intense themselves. We thus expect fewer snowstorms in regions close to the rain-snow line, such as the central United States, although the snowstorms that do occur in those areas are likely to be more intense. It also means we expect more intense snowstorms in generally cold regions. This may appear to be counterintuitive, but the warming to date is not close to that needed to end below-freezing temperatures over large parts of the globe, although it is large enough to put measurably more water vapor into the air.
In addition, warming raises sea levels by heating up and expanding water and by melting landlocked ice in places such as Greenland and Antarctica. Those rising sea levels in turn make devastating storm surges more likely. For instance, warming-driven sea level rise nearly doubled the probability of a Sandy-level flood today compared with 1950. Studies also find that global warming makes the strongest hurricanes more intense, because hurricanes draw their energy from ocean warmth, so that once a hurricane forms, global warming provides it more fuel.