Why 450 ppm is not a safe target


A target of 450 parts per million (ppm) CO2 in the atmosphere is widely regarded as synonymous with keeping mean global temperature by 2100 to no more than 2°C above pre-industrial levels.  This is very misleading and dangerous.  For reasons set out below, achievement of that target, probably by 2030, is likely to result in mean global temperatures dangerously in excess of the predicted 2°C. 

At present we use the concentration of CO2 in the atmosphere as the indicative measure of future temperature.  In doing so, we ignore the effect of other greenhouse gas emissions on temperature.  Why?  Because atmospheric concentration of CO2 is within our control and it is a widely endorsed IPCC finding that the cooling effect of aerosols and changed land use fully off-set the effect of other greenhouse gases. 

That contention has become a convenient convention rather than a reality.  It would be reality were the warming effects of greenhouse gas emissions and the cooling effects of aerosols relatively constant.  But that is not so.  We now know that, as a result of human activity, emissions of other greenhouse gases, particularly methane, are increasing while cooling off-sets are diminishing as countries reduce aerosol emissions to achieve cleaner air.


Although it has a relatively short residence in the atmosphere (~10 years) and low concentration, methane is a particularly dangerous greenhouse gas.  It has much greater capacity to absorb and radiate long wave energy before oxidizing to CO2.

Although produced by “natural” processes such as decay of organic material, most methane is released into the atmosphere as a result of human activity.  This includes methane produced by the massive numbers of farmed animals (cattle, chickens, pigs), coal and other mining, oil refining and, most noticeably, as a feedback from anthropogenic global warming.  The latter is responsible for thawing of permafrost and methane clathrates.

Permafrost, particularly on polar land, has resulted in organic material located on or beneath the surface being frozen.  That stopped the decay process and associated release of methane.  Global warming, particularly polar amplification, is causing permafrost to melt, initially at the surface then to an increasing depth.

This has two effects.  First, buildings and other structures located on frozen land are subject to damage and destruction as permafrost thaws.  Second, organic material also thaws and resumes decaying, releasing methane and other gases in the process.  The quantum of this material and methane produced from thawing of land based permafrost are not known but are thought to be significant.  Emissions from this source are increasing, but do not pose as great a threat as thawing methane clathrate.

Methane continuously seeps from the earths crust.  When it comes into contact with very cold water it forms an ice-like substance known as clathrate.  Normally this substance is stable at depths of 360 meters (m) in the Arctic, though studies in the Svalbad region of northern Norway show that stability is now maintained at depths  >400m, confirming the warming of Arctic Ocean waters.

When it melts, clathrate releases approximately 168 litres of methane for every litre of solid clathrate.  It occurs beneath sediments offshore along the coastline of land bordering the Arctic ocean and has also been found in the Antarctic.  When clathrate melts at depths <400m, methane bubbles to the surface and enters the atmosphere where over a period of 10-12 years it oxidizes to CO2.  Clathrate melting at greater depths usually oxidises to CO2 before it reaches the surface, a process which creates hypoxic conditions, inimical to water breathing animals.

Shakhova (2010) reports that permafrost under the East Siberian Arctic Shelf, is, thawing and starting to leak large amounts of methane into the atmosphere and doing so at an accelerating rate.  She estimates that 1.1 million tones of methane per annum now enters the atmosphere from this source, 3 times as much as is released from on-shore marshlands in this area.  By 2030, those emissions are expected to reach 1.5 gigatonnes/annum.

Consequently, atmospheric presence of methane in the Arctic has now reached the highest it has been for >400,000 years, 1.85ppm compared to 0.7ppm normally found during warm periods.  In parts of the East Siberian Arctic, methane in the atmosphere exceeds 2ppm. This is partly responsible for temperatures in the Arctic rising 2-3 times faster than in the tropics – the so-called Arctic amplification, expected to continue and accelerate the release of methane and ice melt.

Carbon Dioxide

Methane oxidizes to CO2 which has a residence in the atmosphere of ~100 years.  In so doing it reduces the concentration of oxygen in the atmosphere and seawater.  Amounts of CO2 entering the atmosphere from this source will increase as the rate of methane emissions increases.

Vegetation sequesters CO2 from the atmosphere and long-lived plants would normally retain it for several hundred years, possibly longer if the decaying process is arrested through development of anoxic conditions such as burial under sediment.

Rising global temperature is contributing to a large number of trees and other vegetation being killed by insect and other pathogen such as Phytophthora, as well as drought and fire.  Trees which are burned release their stored CO2 immediately.  Those killed by drought and infestation decompose more slowly and take longer to emit CO2.

As temperature continues to increase, damage to vegetation and release of CO2 will also increase.  Forests previously thought of as carbon sinks are becoming a major source of carbon emissions.  Damage to the Amazon rainforest by the 2010 drought is estimated to have caused reduced ability to absorb CO2 and increased emissions resulting in net release into the atmosphere of 2.2 gigatonnes of CO2. Drought events of this magnitude are expected to occur more frequently.

In addition, as surface seawater temperature increases it becomes less able to absorb atmospheric CO2, effectively increasing the amount remaining in the atmosphere.  This is exacerbated by further warming causing the oceans to emit CO2 already stored in seawater.


Human release of chloroflourocarbons (CFCs) prior to enforcement of the Montreal Protocol (1989) caused depletion of stratospheric ozone above the Antarctic and to a lesser extent the Arctic, causing increased exposure to health-damaging ultra-violet radiation.  Ozone depletion also caused cooling of Antarctic surface temperature.

Over the last 20 years of curbing CFC emissions, damage to the ozone layer has been partly reversed and is expected to be fully recovered by 2050.  Recovery will restore the radiative forcing of the ozone layer and have a warming effect globally and particularly over the Antarctic Region.


The radiative effects of aerosols are complex and short-lived but overall, they have a cooling effect.  Increased industrial aerosol emissions 1940–1970 were probably responsible for mich of the slight global cooling trend which occurred in that period.  Since 1970, industrialised nations have significantly reduced aerosol emissions through legislation, regulation and, following collapse of the Soviet Union, by closure of old, high emitting factories.

More recently, industrial aerosol emissions have been reduced by India and China, where they continue to pose a health problem.  As a result, the cooling influence of aerosols has diminished and will continue to do so.


As global temperature rises it causes fast (loss of albedo) and slow (clathrate melting) feedbacks.  A lot of work has been done on examining the effects of fast feedbacks – much less on slow ones, even though their effect on global temperature is becoming increasingly evident.

Slow feedbacks are increasing at an accelerating rate, as are their warming effects.   Stratospheric ozone is increasing reversing the cooling effect caused by its loss, particularly over Antarctica.  Cooling aerosols are diminishing as countries reduce their emissions because of their health effects.

Prior to these developments atmospheric CO2 concentration of 450ppm was equated as limiting average global temperature to 2°C above pre-industrial levels by 2100.  This can no longer be maintained.  Hansen and Sato (2011) using paleoclimate data rather than models of recent and expected climate change warn that “goals of limiting human made warming to 2°C and CO2 to 450 ppm are prescriptions for disaster”.

They predict that pursuit of those goals will result in an average global temperature exceeding those of the Eemian, producing decadal doubling of the rate polar ice loss, resulting in sea level rise of up to 5m by the end of this century.  That prognosis is one which can not be ignored.  Atmospheric CO2 concentration of 450 ppm may be an icon to which politicians and others cling but it is wrong and dangerously so. 

Taking into account all of the above matters, what concentration of CO2 will limit global warming to less than 2°C above pre-industrial levels by 2100?  Hansen suggests 350 ppm, warning that anything above this is dangerous.

Posted by Riduna on Wednesday, 4 May, 2011

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