May 2007 vegetation

Kevin E TrenberthNCAR

Kevin E TrenberthNCAR

Weather and climate in the 21st Century:

What do we know? What don’t we know?

Weather and climate in the 21st Century:

What do we know? What don’t we know?

Global Warming is unequivocalGlobal 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 Temperature1901-2005

Surface Temperature1901-2005

It has not warmed uniformly:More warming over landWhy no warming over SE USA?Or N Atlantic

It has not warmed uniformly:More warming over landWhy no warming over SE USA?Or N Atlantic

IPCC 2007

Extremes of temperature are changing!

Observed trends (days) per decade for 1951 to 2003:

5th or 95th percentiles

From Alexander et al. (2006) and IPCC

Extremes of temperature are changing!

Observed trends (days) per decade for 1951 to 2003:

5th or 95th 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 2002.

The time series (below) accounts for most of the trend in PDSI.

Drought is increasing most placesDrought 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 warmingIncreases in rainfall and cloud counter warming

Trend in Warm Days 1951-2003

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 SoutheastDrought 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

SLP Surface temperature

PrecipitationIPCC 2007

Cooler

Wetter

Pacific Decadal OscillationSST pattern (above) and time series (top right) of 1st EOF of N Pacific SSTs.NPI index of Aleutian LowIndian Ocean SST (tropics)

1976/77 climate shift1976/77 climate shiftIPCC 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 Air holds more water vapor at higher temperatureshigher temperatures

Total water vaporTotal water vapor

Observations show that this is happening at the surface and in lower atmosphere: 0.55C 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: 0.55C 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)

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)

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 2002Changes 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 SSTsNorth Atlantic hurricanes have increased with SSTs

SST(1944-2006)

Marked increase after

1994

IPCC

IPCC AR4

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

2090-2100

(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.