adaptation-focused restorationtechniques are being applied, including revegetating with native species, reducing impervious surfaces, andredundant water distribution systems.

wetter soils can increase the risk of precipitation events causing large floods.

In addition to changing evaporationfrom soil, plants also influence total evaporation through competing effects. In response to increasedatmospheric CO2 concentrations, plant stomata do not need to open as much to take in CO2 for photosyn-thesis, which can reduce the amount of water they lose through transpiration, thus decreasing total evapo-transpiration.340 Conversely, an overall increase in the amount of vegetation can amplify evaporative lossesand surface drying.215,341,342

Key Message 3.5Humans Are Changing Weather and Climate ExtremesHuman activities are affecting climate system processes in ways that alter the intensity,frequency, and/or duration of many weather and climate extremes, including extreme heat,extreme precipitation and flooding, agricultural and hydrological drought, and wildfire (mediumto high confidence).Extreme Heat and ColdChanges in temperature extremes in recent decades are driven primarily by trends toward warmerconditions rather than any changes in variability.180 Consequently, the frequency and intensity of coldextremes have declined over much of the United States while the frequency and intensity of extreme heathave increased.181 Arctic warming may also drive increases in the occurrence and persistence of circulationanomalies that are related to extreme cold and heat,182 although the evidence that these mechanismshave played a role in recent events is mixed.183,184 Climate change may also be contributing to “false spring”events,185 where early warming has caused early budbreak and flowering of plants, exposing them todamaging frost and freeze events.186Extreme Precipitation and FloodingObserved increases in extreme precipitation intensity at the continental scale in North America have been,for the first time, attributed to human influences.187 With warming, it is expected, and has been documented,that more winter precipitation will fall as rain instead of snow,188 although the projected intensity of mostextreme snowstorms remains uncertain.189 Projected increases in extreme precipitation events are largerin the winter season since warming in winter is larger, including events related to atmospheric rivers.190Precipitation associated with hurricanes increases with warming at least as much as water vapor,42 andthe heaviest events can increase at an even greater rate.63,191,192 Where and when increases in extreme pre-cipitation manifest in any given year or even decade is strongly subject to natural variability (see also KM3.3). Notably, increases in extreme precipitation events do not always directly translate to increases inriver flooding, in part because of the many processes at the land surface that affect flood events (KM 4.1;Figure 4.8).DroughtDrought is broadly defined as a transient period of below-average water availability, typically expressedin terms of fluctuations in precipitation, soil moisture, or streamflow and runoff.193 Drought is a complexphenomenon (Figure 3.12) that depends on fluctuations in moisture supply, direct losses of moisture to theatmosphere, and ecosystem and land surface processes.194,195,196 Western North America experienced severaldecadal-scale droughts during the 20th century197 and numerous multidecadal “megadroughts” prior to AD1600198 and is currently experiencing an ongoing megadrought largely unprecedented over the last 1,200years.199,200 See Chapter 6 for more information regarding the effect of changes in the water cycle over land.