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

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/m 2. This corresponds to a 70% increase of CO 2 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 , then a minor cooling followed by a steep warming 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 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

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 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 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 ( C/decade) of tropical lower troposphere (top) Maximum 30-year trend ( C/decade) occurring by chance in a 500 year simulation (below)

Arctic Sea ice mean ice cover indicated

Antarctic Sea ice mean ice cover indicated

Change in Arctic summer sea-ice

Change in Antarctic “ late summer ” sea-ice

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 temperature * Note minor cooling

Ranking of the global warmest years OBS

Ö kningen av v ä xthusgaser och aerosoler leder till en ä ndring i jordens energibalans. Hur kan detta ber ä knas? Hur tillf ö rlitliga ä r dessa ber ä kningar? Hur mycket kan de f ö rb ä ttras? Under de senaste 50 å ren har jordens v ä rmebalans ä ndrats med ca 1.8 W/m 2. Detta motsvarar en ä ndring av solarkonstanten med ca 10 W/m 2. Detta har i detalj ber ä knats och ocks å till dels observerats fr å n satellit. Detta ä r 10 ggr mer ä n solens 11- å rs cykel Effekten av aerosoler har med s ä kerhet reducerat denna uppv ä rmning men kan inte tillf ö rlitligt ber ä knas.

Radiative effects from anthropogenic greenhouse gases.

Radiative forcing of climate (IPCC, 2007)

Atmospheric feedbacks