ATM S 111, Global Warming: Understanding the Forecast

DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES

DAY 6 : 04 /15 /2010

ATM S 111, Global Warming: Understanding the Forecast

Reading Assignment

Should have read “Floods and Droughts” p.58-74

Next assignment: read “The Big Melt” p.75-105

HW 2 due Friday 11:59 PMQuiz #1 is next Tuesday in class

Bring a scantron sheet (available from the bookstore)

Bring a #2 pencil Material will be similar to HW assignments plus

material from this week

Finishing Up Extreme Heat

Dry climates heat up fasterHumidity makes it feel hotter though

And nights stay hotter due to the greenhouse effectSimplest changes with warming: shifting the

distribution Additional expectations: Higher moisture globally

will mean higher heat index as well, and hotter nights If climates dry out, then they will take on more desert-

like characteristicsWhat about really extreme heat?

Worldwide Temperature Extremes

Highest temperature recorded in the world: 136o F (57.8o C) in El Aziziya, Libya

Worldwide Temperature Extremes

Highest in USA: 134o F (56.7o C) in Death Valley, Calif.States with lowest record high: Alaska & Hawaii tie

at 100o F

Highest in Washington: 118o F (48o C) in Grant County

The Gorge Amphitheater in Grant County

Worldwide Temperature Extremes

All these extremes are in desert climatesNote: these are only where weather stations

are located It almost certainly has reached hotter temperatures

elsewhere

Heat Index Extremes

Remember heat index factors in how humid it isHighest heat index in the world:

176o F (80o C) in Dhahran, Saudi Arabia (on the Persian Gulf) Temperature was 108o F (42o

C)

.

Lowest Recorded Temperatures

Coldest ever: -129o F (-89.2o C) in Vostok, Antarctica

Coldest in US: -80o F (-62o C) in Prospect Creek, Alaska

Coldest in lower 48 states: -70o F (-56.7o C) in Montana

Coldest in Washington: -48o F (-44.4o C) in Mazama & Winthrop

Shifting the Distribution of Temperature

If the temperatures just shift to warmer, we’d expect more hot extremes & less cold extremes If the soil dries out, we wouldn’t expect just a shift

though

Houghton textbook

Projected Annual Average Surface Temperature Change: “2080-2099” minus “1980-1999”

Scenario A1B

Projected Soil Moisture Change: “2080-2099” minus “1980-1999”

WetterDrierdrying -> even higher daytime temperaturemoistening -> even higher nighttime temperatures

Scenario A1B

Model Predictions of Extreme Heat: US

Frequency of 20 year heat waves:

1-in-20-year heat waves happen every other year over much of the countryby the end of 21st century

Next few slides from Global Climate ChangeImpacts in the US by US Global Change Research Program, 2009

Model Predictions of Extreme Heat: US

Days over 90o F

Model Predictions of Extreme Heat: US

Days over 100o F

Model Predictions of Extreme Heat: US

Shifting climate zones

Washington State Predictions

Predictions of 2030-2059 compared w/ 1970-1999Change in number of heat waves

(heat wave = 3 straight days w/ heat index over 90o F) Change in number of very warm nights

From Climate Impacts Group report. Other model used has less severe predictions

The future of summer sizzle - the worst heat waves will be more intense - heat waves of a prescribed intensity will occur more frequently - some regions may become more susceptible to heat waves - shift toward higher daytime summer temperatures - vegetated land may give way to desert.

Floods and droughts (RG p. 58-74)Floods and droughts: two sides of a catastrophic coin

A wetter world (but not everywhere)

Are floods increasing?

Defining drought

What do the oceans have to do with drought?

The drying of southern Australia

The plough and its followers: farming and rainfall

Outline

Precipitation around the world Rainy regions and deserts Monsoons

Changes that are expected in the future Wet get wetter, dry get drier Heavy rain events with more heavy rain

Changes that have been observed Increased rainfall? Droughts in the Sahel Droughts in Australia

Rainiest and Driest Places on Earth

Again, these are just places that have weather stations There are likely more extreme places that aren’t being

measured

Rainiest spot on Earth: Mount Waialeale, Hawaii (pictured): 460

inches/year (11.7 meters)Rainiest spot may actually be Lloro, Colombia:

523.6 inches/year (13.3 meters – this is just an estimated amount though)

Rainiest year on record: 905 inches (almost 23 meters) in Cherrapunji, India

In the foothills of the Himalayas

Rainiest day ever: Reunion Island (500 miles off coast of Madagascar)72 inches (1.86 m) in 24 hours!!!

Also just three years ago, Reunion Island set the record for 72 hour rainfall: 155 inches (3.9 m) during a tropical

cyclone

Extreme Precipitation

All of the above examples are associated with mountains More rain on windward side of mountains

When air is lifted up the slope Rain requires water vapor and a lifting mechanism

These extremes are localized rainy areas Just like the Olympic Peninsula here in Washington!

Let’s take a look at the average precipitation over the whole globe There are also large scale patterns of rainy and dry

Observed precipitation in January and July

January

July

Observedprecipitationin mm/yr

Rainiest spots are narrow bands near the equator

Some of the driest places are over the ocean!

Most deserts arearound 30 degreeslatitude

Air rises above the warmest ocean surface(hot air rises)

“Hadley Circulation”

The circulation takes water vaporaway from the deserts at 30 degrees and brings it into the tropical rainy regions

“convergencezone”: where winds come together

Daily = “Sea Breeze” caused by daily solar cycleSeasonally = Monsoon (a persistent version of the sea breeze) caused by seasonal solar cycle

Monsoon Circulation

Heating of land is key: again hot air rises, and circulation brings in water vapor

hotter

colder

Floods and droughts: two sides of a catastrophic coinWet regions become wetter while dry regions

become drier

January

JulyObserved Precipitation

Floods and droughts: two sides of a catastrophic coinWet regions become wetter while dry regions

become drier

January

July

ITCZ ITCZ

ITCZITCZM

M

M

M M

M

SPCZ

SPCZ

SACZ

M=monsoon, CZ=convergence zone: Climate models suggest that these features become stronger as the earth warms.

Climate model projections of precipitation change

Wet gets wetter, dry gets drier

Stippling: where models agree

January

July

As the atmosphere warms, water vapor concentrations increase

Winds bring in even more moisture into the rainy regions

Also more water vapor taken away from dry regions(and more evaporationthere)W = wind from westT = wind from NEblue = rainbelts

Why?

Global Warming Rain Responses

Wet get wetter More water vapor is brought into the regions that are

already rainyDry get drier

More water vapor taken out of the dry regions And more evaporation from dry land surfaces

There’s a lot of uncertainty about specific precipitation responses though Precipitation is much harder to predict than

temperature Regional responses could change significantly from

changes in the winds

Some Additional Predictions

Southwestern North America predicted to dry dramatically As bad as the Dust Bowl by 2060 in some models

Seager et al 2007

Dust Bowl drought level

US Predictions

US predictions

The intensity of downpours is believed to be proportional to water vapor concentration

Warmer temperatures more water vapor

Thus, heavy rainfall events should become more extreme

How about the most intense storms?

1990-2000, Flood Impacts

Nearly 100,000 people were killed 320 million people were displaced by floodsTotal reported economic damages

exceeding a trillion dollars

Bradshaw, et al. Global Change Biol. 13, 1-17 (2007).

http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/accounts/staff/rchambers/GeoBytes/GCSE/Case%20Studies/Causes_of_Flooding_in_Bangladesh.jpg

Do forests prevent flooding?

A recent study says yes: Bradshaw et al., Global Change Biol. 13, 1-17 (2007) Analyzed broader scale data and found deforestation

explained 14% of floods They predict that decreasing forest area by 10%

would increase flood frequency 4-28% and duration 4-8%

Are heavy rain events increasing?

Very heavy events have been increasing

And heavy precipitation events in the US are projected to get worse

Are heavy rain events increasing?

Thus far, the changes are pretty small, but suggestiveExamples of observational results in the scientific literature that suggest an increase on the frequency of heavy rain events.

Note: places where heavy precipitation isn’tincreasing are places where the mean precipitation is decreasing

Are floods increasing?

Trends in the frequency of flood events are difficult to quantify because River configurations and land use are continually changing Hourly rainfall data are available only over limited regions of the globe the data are expensive and there are restrictions on their use Rare events, especially when considered season-by-season it’s difficult to establish statistical significance when dealing with rare events.

The latter two are true for heat waves too

Months or years with below normal water supply. Usually from below average precipitation.

The definition is not quantitative.Specific criteria (e.g., how long, how severe...) need to be specified.Other factors such as population growth can create deficiencies in water supply(ie Lake Chad)

Defining drought

Which way will the Sahel go?

DarfurSUDANthe Sahel

The Sahel is in betweendesert and the region drenched by the African monsoon

The African Monsoon in full swing

Which way will the Sahel go?

wet period extended drought

The shift around 1970 is believed to be due to changing sea surface temperature patterns in the tropical Atlantic (possibly driven by aerosol forcing).

The disappearance of Lake Chad

1972 1987

Lake Chad

The drying of southern Australia

Mean Australian Rainfall (mm)1986 - 1995.

40002400180012001000

800600500400300200150100

0

Desert250

The drying of southern Australia

The decline in rainfall in south-western Australia since the 1960s.Source:http://www.ioci.org.au/publications/pdf/IOCI_Notes_Series2.pdf.

The drying of southern Australia

In most areas (as in SE Australia) the minimum temperature is trending upward faster than the maximum. This is likely because water vapor and/or cloud cover is increasing, which reduce heat loss especially at night. An increase in scattering sunlight could play a role too.

Note that in SW Australia, the drying out causes very high maximum temperature rise too.

The drying of southern Australia

Other factors that may be playing a role

southward shift in the storm track due to the ozone hole

increased water demand due to rising temperatures

increased water demand due to population growth