Climate Impacts by Degree

Global Climate models have been used to calculate various likely future scenarios. Consequences are related not just to the average global temperature but also to the length of time we remain at that temperature. Unfortunately, once C02 is in the atmosphere, its warming effects continue for centuries. In his book and Website Mark Lynas offers a summary of the expected changes by degree.

To date there is no plausible mechanism for extracting atmospheric Co2 in sufficient quantities. Absorption of C02 by forests is massively outpaced by industrial emissions

The most optimistic scenario is that a dramatic change in international targets and treaties and a cap on all fossil fuel extraction causes emissions to start falling in the next ten years. Yet even if we start cutting greenhouse gases today, we are locked in to a 2.89 degree Celsius rise in global temperatures by 2100.

Current levels of warming (less than 1 degree Celsius) seems likely to cause Arctic sea ice to disappear by 2030 or even earlier, causing the extinction or migration of Arctic species such as polar bears.

At 1-2 degrees Summer heat waves will become an annual event, killing tens of thousands and displacing millions of environmental refugees.

Above two degrees of warming – the most optimistic current target – tropical rainforests are likely to release much of their stored carbon via forest fires, see positive feedbacks.

Warmer, more acidic oceans will result in the destruction of coral reefs, and drastic reduction in plankton and krill which are the basis of the marine food chain. This process is already strongly in evidence

At 3-4 degrees Celsius warming, water shortages will strangle downstream cities and agricultural land dries up as glaciers and mountain snow disappears. The most affected areas will be California, Peru, Pakistan and China.

Global food production will be severely reduced as key agricultural areas in Europe, Asia and the United States suffer drought, and heat waves devastate crops.

The most pessimistic scenario is where governments fail to comply with their commitments to climate change.

With no preventative action, there is a 90% probability of a 4.85 C rise by 2100. At 5 degrees Celsius, London and New York would be at risk from flooding, while hundreds of millions of people in China and India would suffer from water shortages as Himalayan glaciers disappear.

At 4-5 degrees Celsius warming, much human habitation in southern Europe, north Africa and the Middle East will be uninhabitable due to excessive heat and drought. Humanity will be forced towards the poles, where temperatures remain cool enough for crops, and the rain still falls, although often in floods.

If we reach 5-6 degrees Celsius of warming, average global temperatures will be hotter than they have been for the last 50 million years. The entire Arctic would be ice-free all year round. Sea levels will rise so rapidly that coastal cities across the world will be abandoned by environmental refugees in their millions.

And past 6 degrees Celsius of warming, the danger of runaway climate change is inevitable, perhaps spurred by the escape of oceanic methane stored below the sea bottom, and released as the deep ocean water becomes increasingly warm. Over time the seas will be nothing but a vast dead zone. The few remaining humans will retreat to highland areas and the polar regions. Perhaps 90% of species will become extinct, beating the worst mass extinctions in the Earth’s 4.5 billion-year history.

These drastic scenarios are predictable with a high degree of confidence. Many of the processes described are already clearly measureable. It seems increasingly likely that we will exceed the critical 2 degrees of warming before the end of this century. This being the case, part of our focus should be on preparing the worst-affected areas to adapt.


Claire, LJ, ‘Global Warming: why is it a bad thing?’, Eco-hopeful, 10 August 2008,

Mark Lynas, ‘Climate change explained – the impact of temperature rises’, The Guardian, 14 April 2009,

Gwynne Dyer, ‘Climate Wars’, 2008, Random House, Canada

Mark Lynas, ‘Climate chaos is inevitable. We can only avert oblivion’, The Guardian, 12 June 2008,

Mark Lynas, ‘Climate change explained – the impact of temperature rises’, The Guardian, 14 April 2009,

Claire, LJ, ‘Global Warming: why is it a bad thing?’, Eco-hopeful, 10 August 2008,

Mark Lynas, ‘Climate change explained – the impact of temperature rises’, The Guardian, 14 April 2009,

Steve Holland and Mark Lynas, ‘6 degrees’,