What happens if the world warms up by 3°C?

A 3°C temperature rise may be the tipping point where
global warming could run out of control, leaving us powerless to intervene as
planetary temperatures soar. America’s most eminent climate scientist, James Hansen
says warming has brought us to the “precipice of a great tipping
point”. If we go over the edge, it will be a transition to a different
planet, an environment far outside the range that has been experienced by
humanity. There will be no return within the lifetime of any generation that
can be imagined, and the tip will exterminate a large fraction of species on
the planet.

In the Pliocene, three million years, temperatures
were 3°C higher than our pre-industrial levels, so it gives us an insight into
the 3°C world. The northern hemisphere was free of glaciers and icesheets,
beech trees grew in the Transantarctic mountains, sea levels were 25 metres
higher, and atmospherc carbon dioxide levels were 360-400 ppm, very similar to
today. There are also strong indications that during the Pliocene, permanent El
Nino conditions prevailed. Hansen says that rapid warming today is already
heating up the western Pacific Ocean, a basis for a coming period of
“super El Ninos”.

Between two and three degrees temperature rise the
Amazon rainforest, whose plants produce 10% of the world’s
photosynthesis and have no evolved resistance to fire, may turn to savannah, as
drought and mega fires first destroy the rainforest, turning trees back into
carbon dioxide as they burn or rot and decompose. The carbon released by the
forests destruction will be joined by still more from the world’s soils,
together boosting global temperatures by a further 1.5ºC. It is suggested than
in human terms the effect on the planet will be like cutting off oxygen during
an asthma attack.


A March 2007 conference at Oxford talked about “corridors of probability” with models predicting the risk of the Amazon passing a tipping point at between 10 to 40% over the next few decades. The UK’s Hadley Centre climate change model, best known for warning of catastrophic losses of Amazon forest, predicts that, under current levels of greenhouse gas emissions, the chances of such a drought would rise from 5% now (one every 20 years) to 50% by 2030, and to 90% by 2100.

The collapse of the Amazon is
part of the reversal of the carbon cycle projected to happen around 3°C, a view
confirmed by a range of researchers using carbon coupled climate models. Vast
amounts of dead vegetation stored in the soil (more than double the entire
carbon content of the atmosphere) will be broken down by bacteria as soil
warms. The generally accepted estimate is that the soil carbon reservoir
contains some 1600 gigatonnes, more than double the entire carbon content of the
atmosphere. The conversion will begin of the terestrial carbon sink to a carbon
source due to temperature-enhanced soil and plant respiration overcoming
CO2-enhanced photosynthesis, resulting in widespread desertification and
enhanced feedback.

And it’s already happening. A recent study found that the calculated increase
in carbon lost by UK soil each year since 1978 is more than the entire
reduction in emissions the UK has achieved between 1990 and 2002 as part of its
commitment to the Kyoto Protocol. New research published in “Science”
in May 2007 suggests that the earth’s ability to soak up the gases causing
global warming is beginning to fail because of rising temperatures, in a
long-feared sign of “positive feedback”.

Three degrees would likely see increasing areas of the planet being rendered
essentially uninhabitable by drought and heat. Rainfall in Mexico and central
America is projected to fall 50%. Southern Africa would be exposed
to perennial drought, a huge expanse centred on Botswana could see a
remobilisation of old sand dunes, much as is projected to happen earlier in the
US west. The Rockies would be snowless and the Colorado river will fail half
the time. Drought intensity in Australia could triple, according to the CSIRO,
which also predicts days in New South Wales above 35°C will increase 2 to 7

– Read also “If the world warms up by 2°C”: http://adastra.jeandar.net/#post442