Why is the global temperature not increasing more rapidly? Lennart Bengtsson ISSI ESSC.

Why is the global temperature not increasing more rapidly?

Lennart BengtssonISSIESSC

Why is the global temperature not increasing more rapidly?

• The total effect of greenhouse gas emission on the global radiation balance since 1900 is equal to ca 2.5W/m2.

• This corresponds to a 70% increase of CO2 as the effect is proportional to the logarithm of the concentration.

• At the same time the surface of the global oceans have warmed ca 0.6°C

• Over land areas the warming is about 0.1°C higher

The evolution of global warming

• The warming in time is irregular with a rapid warming 1920-1945, then a minor cooling 1945 -1975 followed by a steep warming 1975-2000.

• After 2000 there has not been any further warming.

• In the same way the warming varies geographically with the highest warming at high latitudes of the Northern Hemisphere but in fact a minor cooling around Antarctica.

What is the general view communicated to the public

• The situation with climate warming is becoming more serious and is in fact much worse than was reported in the IPCC report from 2007.

• Artic ice is melting more rapidly then expected

• The expected losses of the land ices might lead to a rise in sea level by about 1m at the end of this century.

• There will be more cyclones in the future and they will be more intense.

What do we actually observe?

• During the last 5 years the sea level is rising more slowly and the expansion of sea water only contribute some 20%, the rest is melt water and net water transport from land.

• The global surface temperature has stayed more or less constant the last 10 years.

• While sea ice in the Arctic has diminished markedly but the sea ice around Antarctica has been increasing during the last three decades.

• Recent studies have shown that the increase in tropical cyclones in the Atlantic is rather a consequence of better observations.

• Mid latitude storms has not increased in intensity and are not increasing in number.

Surface temperature 1850-2009

Global temperature change

• The surface temperature of the Earth has never been constant, but has undergone changes, sometimes rapid, on a multitude of time scales, many of which are incompletely understood. Since 1900, the global mean surface temperature has increased by 0.6 to 0.8 °C. The warming has not been regular, but has rather occurred in steps that might be interpreted as multi-decadal fluctuations superimposed on a long-term warming trend.

• What is the picture during the last 30 years?• Let us inspect satellite micro wave measurements now

covering 30 years.

Observed temperature trend for the lower troposphere 1979-2008

Temperature of the lower troposphere, TLT, from MSU

• TLT represent the temperature between the surface and ca. 7 km above the ground, most of the radiation comes from the area below 3 km.

• The marked warming peak is related to the El Nino event 1997/98

Observed temperature trend for the lower troposphere 1979-2008

Note the larger warming at higher latitudes of the Northern Hemisphere where climate forcing is

smallest (reflecting aerosols that are less common at the Southern Hemisphere).

Observed trend (+0.149 C/decade) of tropical lower troposphere 1979-2008 (top)

Maximum 30-year trend (+0.220 C/decade) occurring by chance in a 500 year simulation

(below)

Arctic Sea ice 6.5. 2009 mean ice cover 1979-2000 indicated

Antarctic Sea ice 6.5. 2009mean ice cover 1979-2000 indicated

Change in Arctic summer sea-ice 1979-2007

Change in Antarctic “ late summer” sea-ice 1979-2007

What we need to know better

• To what extend are observed changes due to natural processes?

• What are the mechanisms behind the natural processes?

• Could it be that the changes are due to albedo changes caused my anthropogenic aerosol emission?

• In the 1950s and 1960s there was a rapid increase in sulphate aerosols emission due to the industrialization and increasing energy production.

• Presently, the second wave of industrialization dominated by China and India could be the cause of the present reduced warming

The global temperature1850-2008

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* Note minor cooling

Ranking of the global warmest years 1850-2008

OBS

Ökningen av växthusgaser och aerosoler leder till en ändring i jordens energibalans. Hur kan detta beräknas? Hur tillförlitliga är dessa beräkningar? Hur mycket kan

de förbättras?

• Under de senaste 50 åren har jordens värmebalans ändrats med ca 1.8 W/m2. Detta motsvarar en ändring av solarkonstanten med ca 10 W/m2. Detta har i detalj beräknats och också till dels observerats från satellit.

• Detta är 10 ggr mer än solens 11-års cykel

• Effekten av aerosoler har med säkerhet reducerat denna uppvärmning men kan inte tillförlitligt beräknas.

Radiative effects from anthropogenic greenhouse gases.

Radiative forcing of climate 1750-2005 (IPCC, 2007)

Atmospheric feedbacks

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