Climate scientists are trying to define the level of risk associated with future climate change ā although they leave it up to others to decide how much risk is too much.
Defining dangerous climate change is tricky because it begs the question ādangerous to whom?ā, says Joel Smith of environmental research firm in Boulder, Colorado.
To tackle the problem, in 2001, Smith gathered together some leading scientists, each specialising in a different aspect of climate change, such as species extinction and the cost of adaptation. Together they drew up what is now known as the āburning embersā diagram (see graphic, right).
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The diagram shows how different risks increase as temperatures rise. There are five embers, each corresponding to a major risk:
ā¢ the risk to unique ecosystems ā eg the Arctic sea ice ā and endangered species
ā¢ the risk of extreme weather events
ā¢ the human extent of the risk ā whether it affects some societies or many
ā¢ the economic and health risks
ā¢ the risk of hitting ātipping pointsā beyond which change progresses much faster and is largely irreversible
The expert judgement of his collaborators was drawn upon to illustrate how risk would grow with temperature in each category, says Smith. āThere is no formula that we use. I think itās possible that another set of experts would come up with a different set of temperatures.ā
Climate impacts
Smith has now reassembled the team to update the graph. Their conclusion, not surprisingly, is that the risks have increased.
āWe were surprised how quickly impacts that can be attributed to climate change are becoming evident,ā says Smith. He mentions in particular an increase in the frequency and intensity of extreme events, and increases in the number of wildfires.
Overall, the new graph shows that climate change poses a greater risk at lower temperatures than estimated in 2001.
For some categories, it suggests the planet is already close to being in the red. In particular, the amount of warming that we are currently experiencing is deemed to already pose significant risk to unique ecosystems and species.
āSignificant impactsā
The conclusions are intentionally fuzzy, says Smith. āItās not as if we would go from 0.99 Ā°C to 1 Ā°C and suddenly all unique ecosystems would disappear. The graph conveys a sense that somewhere around that [figure] we will start to see very significant impacts.ā
Most governments have adopted the target of not allowing the planet to become more than 2Ā° C warmer than it was before the 19th century industrial revolution. Journalists and politicians tend to say that beyond 2 Ā°C is ādangerousā.
The planet is already 0.74 Ā°C warmer on average than it was in pre-industrial times. Smithās graph shows that the further 1.26 Ā°C of warming would land us squarely in the red for the first three categories of risk, suggesting, says , of Penn State University, that we are running out of time.
Journal reference: (DOI: 10.1073/pnas.0812355106) (pdf)
