Climate change and the weather

Halldór Björnsson

Weather vs climate

Edward Lorenz (20th century): “Climate is what you expect and weather is

what you get”

Myles Allen (21st century): “Climate is what you change, and weather is

what gets you”

This weather is brought to you by ....

During almost any notable weather event (except maybe extra cold snaps) a meteorologist will be asked: Is this global warming?

Possible exception will be when next big El Nino strikes...

Is this rain/wind/heat/cold/cloud pattern/bird migration....due to El Nino?

Do such questions make sense?

Weather --> Climate Climate is the ensemble weather

Described by long term statistics (averages & distribution) of wind, temperature, precipitation etc

Climate is the envenlope of possible weathers Edward Lorenz referred to Climate as the

Weather Attractor Implying that Climate was made up of all possible

states of the weather

But does it work the other way?

Climate --> Weather

Climate can also be though of as the set of conditions that determine what weather is possible Conditions such as ocean, ocean currents,

ice, chemical composition, snow cover, vegitation, orography, etc

From this viewpoint it is easy to see climate change impacting weather. --> The climate system

The climate system

Controls on the climate system

Greenhouse effectRadiative forcing

The greenhouse effect

Its been known for a long time that the atmosphere keeps the surface of the earth warmer than direct solar heating implies.

This is primarily due to a few different gasses present in the atmosphere.... .....some in only in very small quantities

Fourier (1827)

In a rather incomprehensible article Fourier writes: ... c’est ainsi que la température est

augmentée par l’interposition de l’atmosphère, parce que la chaleur trouve moins d’obstacle pour pénétrer l’air, étant a l’etat de lumière, quelle n’en trouve pour repasse dans l’air lorsque’elle est convertie en chaleur obscure.

Tyndall (1861) Tyndall measured what trace gasses

where responsible for the absorbtion of infra-red radiation, he writes:

It is not, therefore, necessary to assume alterations in the density and height of the atmosphere to account for different amounts of heat being preserved to the earth at different times; a slight change in its variable constituents would suffice for this. Such changes in fact may have produced all the mutations of climate which the researches of the geologists reveal.

Chief gasses he found are CO2 and H2O

The effect can be measured

CO2

O3

H2O

H2O

Weather satellites measure heat radiation Those measurements clearly show the

greenhouse effect

At noon on this day the US weather satellite NIMBUS 4 went over the Iceland

Greenhouse gasses are on the rise.. CO2 concentration

have gone up by 100ppm (36%) in the last 250 years Half of the

increase in the last 50 years

Beyond doubt that this is due to human activity

Other greenhouse gasses also increasing

IPCC & the 4th assessment

To ensure that the community of nations had reliable information on the issue the UN founded the Intergovernmental Panel on Climate Change (IPCC) in 1988.

The panel publishes assessments on the state of knowledge in the field. The 4th assessment was published in 2007

In the 70's the increasing greenhouse gasses led to worries of impending global warming

1973 was the warmest year since 1938

A list of 10 warmest years includes only 1 year prior to 1998, and all the years since 2001 are on it.

Update: 2008 took spot number 10. No year prior to 1998 left on the top 10 list.

1981 was warmer, then came another record in 1983, and that record lasted until 1988, then came 1990, 1995, and 1998 was unprecedented ..... until matched by 2005!

Radiative forcing since pre-industrial

Changes in CO2have largest impact

Some changes havea cooling effect

Influence of solar variability is small

A definite increase

=>Global warming

Fig AR4 WGI TS.5

Changes in radiative forcing

Warming of 0.7°C during last 100 years

Sea level rise

Decreasing snow cover in N-Hemisphere

Anthropogenic?

Only when human induced increases in greenhouse gasses and aerosols are included in climate models can they simulate temperature changes during the last century

IPCC 2007 recap Warming in last 100 year slightly exceeds

0.7°C Warming intensified towards northern high

latitudes Diminished snow cover, earlier spring thaw Widespread glacier retreat Reduced sea ice cover, especially during

summer Sea level rise

1.8 cm/decade since 1961 & 3.1 cm from 1993 to 2003

Ecosystems: poleward and upwards shift in ranges for various plants & animals Earlier spring events (egg laying, leaf

unfolding etc.)

IPCC 2007 recap cont. Precipitation signal is complex:

More precipitation in wet tropics and higher latitudes.

Less precipitation in many areas that are already dry.

More intense rain events, - even in areas where droughts are more common.

Frequency of floods and droughts has increased.

Higher incidence of heat related fatalities Oceanic acidification.

Top right: Crop failure due to droughts in Somalia

Left: Scenes from Dhaka, Bangladesh.

Seasonal flooding getting worse

The human face of climate change

Quo vadis?Two degrees since pre-industrial

21st Century warming

2 decades hence End of days

Scenario

B1

A1B

A2

Greater warming over continents than oceanic areas

Enhanced polar warming

(IPCC 2007)

Sea level rise

The exclusion of rapid changes in ice flow is controversial

If changes scale with temperature increase then 10 – 20 cm can be added to the sea level numbers

Higher values (1m ?) may be justified (Rahmstorf 2007)

And the weather?

Changes in mean & extreme

Weather (temperature, precip, wind etc)

Inci

dence

Distribution shifted to higher values

Distribution shifted and distorted

The distribution of a weather related parameter may be shifted

e.g. uniform warming

The distribution may be shifted and distorted

More changes in extremes

More often strange weather...

When can you say strange weather is due to global

warming? Short answer:

Never!!!!

Is that correct? Probably not....

It is unlikely that you can say with certainty, but certain kinds of weather may fit-the-bill...

... made more likely by global change

Often the question really is: is our fault?

The summer of 2003 in Europe

The heatwave had complex causes Persistent clear skies, dry soils etc.

Model simulations show that anthropogenic greenhouse effect doubled the risk of such a hot summer in Europe

(Switzerland 1864 - 2003)

Hurricane Katrina The costliest hurricane in history.

But note: Bhola killed ~ ½ million in Bangladesh

Was it due to global warming? Warm waters make stronger hurricanes

The loop current was unusually warm Both Rita and Katrina intensified on passing over

the loop current Was the loop current exceptionally warm? Probably not.

Climate change will probably lead to stronger hurricanes.

Contrast with El Nino During El Nino certain

temperature and precipitation anomalies are well established

Can you pinpoint individual storms on El Nino? In California a forcaster

may say: “Its El Nino season, and

then we get these kinds of storms”

Tem. MAM. Red:Hot

Precip. MAM. Red: Dry

Source: KNMI

Summary

Climate is both the statistics of the weather and the system that sets bounds on what weather is “normal”

Climate change is a change in weather statistics (mean & extremes)

Individual events are not likely to be pinpointed as being due to climate change

However, certain types of weather become more common with climate change

Everything radiates Everything warmer than 0K radiate

energy Example: Solar radiation, we see part of it.

Solar energy heats the surface

If this was all then the globewould be a lot colder at least 33°C colder

Greenhouse effect The surface warms and radiates heat Clouds and atmospheric gasses absorb

the radiation, warm and radiate... into space .. ..and towards the

surface As a consequence the

lower part of the atmosphere and the surface warm

This is known as the greenhouse effect

Radiative forcing