Climate Basics Explained: How hot is too hot?

Today the world’s average temperature is just under one degree Celsius above what it was in pre-industrial times. Some scientists are saying that with the current level of greenhouse gases in the atmosphere, a two degree rise is locked in unless we can reduce emissions to almost zero and actively remove carbon from the atmosphere. It is also likely that on current emissions trajectories, we will reach a six degree rise by 2100.

The Australian Federal Government and most governments around the world have yet to officially set what temperature rise they believe is adequately safe. Many unofficially subscribe to the aim of keeping temperature rise to two degrees or below. However, the most recent science indicates that this may be too high and trigger feedback loops that result in more temperature rises outside of humanity’s control.

So what do these temperature rises mean in reality?

The following summary has been taken from two sources. Mark Lynas’ article in The Guardian and David Spratt’s presentation at the Moreland Climate Group‘s recent climate debate. We will count down from six to one.

Six Degrees and Above
Danger of “runaway warming”, perhaps spurred by the release of oceanic methane hydrates. Could the surface of the Earth become like Venus, entirely uninhabitable? Most sea life is dead. Human refuges are now confined entirely to highland areas and the polar regions. The human population is drastically reduced. Perhaps 90% of species become extinct, rivalling the worst mass extinctions in the Earth’s 4.5 billion-year history.

Five Degrees
Global average temperatures are now hotter than for 50 million years. At this time breadfruit trees grew on the coast of Greenland, while the Arctic Ocean saw water temperatures of 20 degrees Celsius. There was no ice at either pole (today that means a 70-metre sea-level rise), and much of the world would have been desertified.

At five degrees, most of the topics, sub-tropics and even lower mid-latitudes are too hot to be inhabitable. Sea level rise is now sufficiently rapid that coastal cities across the world are largely abandoned.

Prof. Hans Joachim Schellnuhuber, Director of the Potsdam Institute and adviser to the European Union and to the German Chancellor, told the Copenhagen science conference in March that a rise to 5–6 degrees Celsius above preindustrial levels would reduce “the carrying capacity of the planet (to) below 1 billion people”.

Four Degrees
A tipping point sees massive amounts of methane – a potent greenhouse gas – released by melting Siberian permafrost, further boosting global warming and making further human action to mitigate emissions futile. Much human habitation in southern Europe, north Africa, the Middle East and other sub-tropical areas is rendered unviable due to excessive heat and drought. Deserts are spreading in Italy, Spain, Greece and Turkey. The focus of civilisation moves towards the poles, where temperatures remain cool enough for crops, and rainfall – albeit with severe floods – persists. All sea ice is gone from both poles; mountain glaciers are gone from the Andes, Alps and Rockies, with severe water supply implications for these areas.

Three Degrees
3 degrees Celsius is the cap effectively being advocated by Australia’s Labor government. Labor policy is a 60 per cent reduction in Australian emissions by 2050. Sir Nicholas Stern says explicitly that for developed nations this is a 3 degrees Celsius target, telling the National Press Club in Canberra it would be “a very good idea if all rich countries, including Australia, set themselves a target for 2050 of at least 60 per cent emissions reductions” and this would leave us with “roughly a ?fty-?fty chance of being either side of 3 degrees above pre-industrial times”.

This is the target that both Stern and Garnaut advocated, but Stern now says that “We haven’t seen 3 degrees Celsius for a few million years, and we don’t know what that looks like”. But from the Pliocene 3 million years ago we know what a 3 degrees Celsius world would likely be: a northern hemisphere free of glaciers and icesheets, where beech trees grew in the Transantarctic mountains, sea levels were 25 metres higher, and probably permanent El Nino conditions.

Glacier and snow-melt in the world’s mountain chains will deplete freshwater flows to downstream cities and agricultural land. Most affected are California, Peru, Pakistan and China. Global food production is under threat as key breadbaskets in Europe, Asia and the United States suffer drought, and heatwaves outstrip the tolerance of crops. The Gulf Stream current declines significantly. Cooling in Europe is unlikely due to global warming, but oceanic changes alter weather patterns and lead to higher than average sea level rise in the eastern US and UK.

NASA climate chief Dr James Hansen has warned that a 3 degrees Celsius warming “threatens even greater calamity, because it could unleash positive feedbacks such as melting of frozen methane in the Arctic, as occurred 55 million years ago, when more than 90 per cent of species on Earth went extinct”. Hitting three degrees may mean that we are not able to stop there.

Two Degrees
2 degrees Celsius has been a target of convenience in international negotiations, but is now losing consensus as the politicians head to 3 and 4 degrees Celsius, and the scientists towards zero.

To have a 2 in 3 chance of holding to 2 degrees Celsius, atmospheric carbon needs to be held to 400ppm CO2e (carbon dioxide equivalent) and that requires a global reduction is emissions of 80% by 2050 (over 1990) and negative emissions after 2070. For Australia, a 2 degrees Celsius target means a more than 95% cut by 2050.

A rise of 2 degrees Celsius over pre-industrial temperatures will initiate large climate feedbacks in the oceans, on ice-sheets, and on the tundra, taking the Earth well past signi?cant tipping points. Likely impacts include large-scale disintegration of the Greenland and West Antarctic ice-sheet; the extinction of an estimated 15– 40 per cent of plant and animal species; dangerous ocean acidi?cation; increasing methane release; substantial soil and ocean carbon-cycle feedbacks; and widespread drought and deserti?cation in Africa, Australia, Mediterranean Europe, and the western USA.

Hansen told the US Congress last year that: “We have reached a point of planetary emergency… climate is nearing dangerous tipping points. Elements of a perfect storm, a global cataclysm, are assembled… the oft-stated goal to keep global warming less than +2 degrees Celsius is a recipe for global disaster, not salvation.”

Summer heatwaves such as that in Europe in 2003, which killed 30,000 people, become annual events. Extreme heat sees temperatures reaching the low 40s Celsius in southern England. Amazon rainforest crosses a “tipping point” where extreme heat and lower rainfall makes the forest unviable – much of it burns and is replaced by desert and savannah. Dissolved CO2 turns the oceans increasingly acidic, destroying remaining coral reefs and wiping out many species of plankton which are the basis of the marine food chain. Several metres of sea level rise is now inevitable.

One Degree
Today at just less than 1 degree Celsius of global warming we are witnessing of the destruction of the Arctic ecosystem. Eight million square kilometres of sea ice is disappearing fast each summer and may be entirely gone within a few years. Already 80% by volume of summer sea-ice has been lost, and regional warming of up to 5 degrees Celsius may have already pushed the Greenland ice-sheet (eventual sea-level rise of 7 metres) past its tipping point.

At less than 1 degree Celsius there is more frequent and intense heatwaves, ongoing drought around the Mediterranean and in Australia, sub-Saharan Africa and the western US, and the swift retreat of river-feeding mountain glaciers. The eastern Amazon is drying (some tributaries ran dry in the 2005 drought), low-lying island states are on the edge of a precipice, as are coral reefs. Britain’s Hadley Centre calculates that warming of just 1C would eliminate fresh water from a third of the world’s land surface by 2100.

It is obvious from these predictions that we need to reduce emissions and draw down carbon from the atmosphere, to get warming back down to as close to zero as possible. We need to make politically possible what is scientifically necessary.