Heat Waves

Climate change amplifies the intensity, duration and frequency of heat wave events. Even a small change in average global temperature can lead to a dramatic change in the frequency of extreme events such as heat waves.

Since 1950, the number and duration of heat waves worldwide has increased, as has the frequency of hot days and nights (IPCC AR5 WGI, Table SPM.1) and the level of humidity in the air (Willett et al. 2007). The geographic area hit by extreme summer temperatures has grown by well over ten times in the past 30 years (Hansen et al. 2012).

The influence of human-caused global warming has been firmly identified in all of these trends (IPCC AR5 WGI, Table SPM.1; Willett et al. 2007; Hansen et al. 2012).

Extreme temperatures have an element of probability. Climate change provides baseline warming that raises the bar on what natural variation can produce. For example, one study found that greenhouse gas pollution caused “over half” of the anomalous U.S. warming of 2006 (Hoerling et al. 2007), and another found that Russia’s devastating 2010 heat wave was made over five times more likely by climate change (Watanbe et al. 2013). In recent years, new record-breaking high temperatures have outnumbered new record lows in the U.S. by a ratio of about 2:1 (NCA Chpt 2 Fig 2.18, p.53).

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Cold Spells

Average global temperatures are rising with global warming. Natural variation means that we will still sometimes see cold spells, but they are becoming less frequent. In some cases, cold spells might be connected to warming if they are associated with weather patterns such as increased “waviness” in the jet stream. Warming of the Arctic may be slowing the jet stream, which enables larger waves to form and allows cold air to move farther south than usual. These large jet stream waves were thought to be involved in the cold weather that covered the Eastern U.S. in early 2014.

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