May 2007 vegetation Kevin E Trenberth NCAR Kevin E Trenberth NCAR Weather and climate in the 21 st Century: What do we know? What don’t we know?

Global Warming is unequivocal Since 1970, rise in:Decrease in:  Global surface temperatures NH Snow extent  Tropospheric temperatures Arctic sea ice  Global SSTs, ocean Ts Glaciers  Global sea level Cold temperatures  Water vapor  Rainfall intensity  Precipitation extratropics  Hurricane intensity  Drought  Extreme high temperatures  Heat waves Since 1970, rise in:Decrease in:  Global surface temperatures NH Snow extent  Tropospheric temperatures Arctic sea ice  Global SSTs, ocean Ts Glaciers  Global sea level Cold temperatures  Water vapor  Rainfall intensity  Precipitation extratropics  Hurricane intensity  Drought  Extreme high temperatures  Heat waves IPCC 2007

Surface Temperature Surface Temperature It has not warmed uniformly: More warming over land Why no warming over SE USA? Or N Atlantic It has not warmed uniformly: More warming over land Why no warming over SE USA? Or N Atlantic IPCC 2007

Extremes of temperature are changing! Observed trends (days) per decade for 1951 to 2003: 5 th or 95 th percentiles From Alexander et al. (2006) and IPCC Extremes of temperature are changing! Observed trends (days) per decade for 1951 to 2003: 5 th or 95 th percentiles From Alexander et al. (2006) and IPCC

The most important spatial pattern (top) of the monthly Palmer Drought Severity Index (PDSI) for 1900 to The time series (below) accounts for most of the trend in PDSI. Drought is increasing most places Mainly decrease in rain over land in tropics and subtropics, but enhanced by increased atmospheric demand with warming IPCC 2007

Absence of warming by day coincides with wetter and cloudier conditions Drought Increases in rainfall and cloud counter warming Trend in Warm Days IPCC 2007

May 2007 vegetation US changes in Precipitation Temperature Much wetter 1930s: Hot and dry

PDSI: severe or extreme drought Change in area of PDSI in drought using detrended temperature and precipitation: Red is no trend in precipitation: Would be much more drought! Blue is no trend in temperature. Modest warming has contributed Easterling et al 2007 The warmer conditions suggest that drought would have been much worse if it were not for the much wetter conditions. And it would have been much warmer too!

May 2007 vegetation Recent drought in SE US was a reversal of patterns for last two decades: One that could become much more common Drought in Southeast

Regional climate change Hypothesis: It is impossible to address regional climate change without fully addressing how patterns of climate variability (modes) change, and thus how: ENSO: El Niño Southern Oscillation NAO/NAM: North Atlantic Oscillation/Northern Annular Mode SAM: Southern Annular Mode PDO: Pacific Decadal Oscillation AMO: Atlantic Multidecadal Oscillation change! Regional climate change Hypothesis: It is impossible to address regional climate change without fully addressing how patterns of climate variability (modes) change, and thus how: ENSO: El Niño Southern Oscillation NAO/NAM: North Atlantic Oscillation/Northern Annular Mode SAM: Southern Annular Mode PDO: Pacific Decadal Oscillation AMO: Atlantic Multidecadal Oscillation change!

El Niño - Southern Oscillation SLPSurface temperature Precipitation IPCC 2007 Cooler Wetter

Pacific Decadal Oscillation SST pattern (above) and time series (top right) of 1 st EOF of N Pacific SSTs. NPI index of Aleutian Low Indian Ocean SST (tropics) 1976/77 climate shift IPCC 2007

Extremes in climate Changes in extremes matter most for society and the environment  With a warming climate:  More high temperatures, heat waves  Wild fires and other consequences  Fewer cold extremes.  More extremes in hydrological cycle:  Drought, heavy rains, floods

Air holds more water vapor at higher temperatures Total water vapor Observations show that this is happening at the surface and in lower atmosphere: 0.55  C since 1970 over global oceans and 4% more water vapor. This means more moisture available for storms and an enhanced greenhouse effect. Observations show that this is happening at the surface and in lower atmosphere: 0.55  C since 1970 over global oceans and 4% more water vapor. This means more moisture available for storms and an enhanced greenhouse effect. A basic physical law tells us that the water holding capacity of the atmosphere goes up at about 7% per degree Celsius increase in temperature. (4% per  F)

Role of character of precipitation: Daily Precipitation at 2 stations Frequency 6.7% Intensity 37.5 mm Frequency 67% Intensity 3.75 mm Monthly Amount 75 mm drought wild fires local wilting plants floods soil moisture replenished virtually no runoff ABAB

Changes in total, heavy, and very heavy precipitation over contiguous U.S. Linear trends are up and significant at 1%: 7, 14, 20% /century Groisman et al 2004 Changes in total, heavy, and very heavy precipitation over contiguous U.S. Linear trends are up and significant at 1%: 7, 14, 20% /century Groisman et al 2004 Changes in U.S. precipitation 1900 to 2002

N. Atlantic hurricane record best after 1944 with aircraft surveillance. Global number and percentage of intense hurricanes is increasing North Atlantic hurricanes have increased with SSTs SST ( ) Marked increase after 1994 IPCC

IPCC AR4

Combined effects of increased precipitation intensity and more dry days contribute to mean precipitation changes

( Tebaldi, C., J.M. Arblaster, K. Hayhoe, and G.A. Meehl, 2006: Going to the extremes: An intercomparison of model-simulated historical and future changes in extreme events. Clim. Change.)

May 2007 vegetation Rising greenhouse gases are causing climate change, and arid areas are becoming drier while wet areas are becoming wetter. Water management:- dealing with how to save in times of excess for times of drought – will be a major challenge in the future. Rising greenhouse gases are causing climate change, and arid areas are becoming drier while wet areas are becoming wetter. Water management:- dealing with how to save in times of excess for times of drought – will be a major challenge in the future.