Climate change poised to feed on itself
Fifteen of Australia's top climate experts explain how we know humans are altering the atmosphere and why we must act now.
Around the world, thousands of scientists have devoted their professional lives to studying the climate. Not centrally organised, they sometimes build temporary affiliations but they remain scientists throughout – that is, they are independent, constantly challenge each other and are committed to searching for truth through objective, independently verifiable evidence.
Overwhelmingly, this evidence has led to four conclusions. The first is that the world is warming. The global average temperature has increased by about 0.8 degrees since 1850, with most of the increase occurring since 1950. The warming varies among decades because of natural fluctuations but the overall trend has been inexorably upward.
Warming is evident in other indicators, such as rising sea levels and reduced sea-ice and snow cover. Of these, the most important measure is the extra heat in the oceans, which is steadily rising. Claims that climate change has reversed since 1998 are misrepresentations of the full data.
The second conclusion is that the dominant cause of the warming since about 1950 is the increase in the atmospheric concentrations of greenhouse gases released by human activities, of which carbon dioxide (CO2) is the most important.
This critical conclusion is based on several independent lines of evidence, including basic physics, studies of climate changes in both in the geological past and in the industrial era, and finally – but far from solely – from the predictions of climate models. Together, these provide an overwhelming case that increasing greenhouse gas concentrations cause warming, and that CO2 is the largest contributor to the current warming trend.
Other contributors include changes in the sun’s output associated with sunspots and solar flares, and volcanic dust. However, if these were solely responsible for temperature changes since 1850, the world should have cooled over the past half-century rather than warming at an increasing rate.
The third conclusion is that warming will increase in future, if emissions of CO2 and other greenhouse gases maintain their present paths. "Business as usual" scenarios for future emissions lead to likely global temperature increases of up to six degrees above present temperatures by 2100.
These are dramatic temperature increases, which would be accompanied by major disruptions to food supplies, river flows and water availability, significant and ongoing rises in sea level (of up to about a metre by 2100 and potentially metres over longer times), disease threats, disruptions to ecosystems including the extinction of many species, and social and geopolitical destabilisation.
The fourth conclusion is that climate change cannot be reversed for many centuries, because of the massive heat stores in the world’s oceans. Even if CO2 and other greenhouse gas concentrations were stabilised today at their present levels, a further warming of at least 0.6 degrees would inevitably follow (on top of the 0.8 degrees observed since 1850) and sea-level rise would continue for centuries to millenniums.
These four conclusions have been known and agreed among thousands of independent climate scientists for more than a decade. However, new findings suggest that the situation is, if anything, more serious than the assessment of just a few years ago.
The heightened concern among climate scientists arises from a growing realisation that climate change can be accelerated beyond current predictions by reinforcing "climate feedbacks", which contribute to climate change and are accelerated as it occurs, thus causing climate change to feed on itself. When these feedbacks are sufficiently strong they become "climate tipping points" which can flip the climate into a new state with essentially no way to recover.
Several feedbacks are of immediate concern. Interactions between climate and the earth’s carbon cycle (the exchange of carbon between the atmosphere, the land and the oceans) will act to accelerate climate change if sinks do not keep pace with emissions (as is already happening) and/or if previously stable carbon stores are released to the atmosphere under climate change, for example by the thawing of carbon-rich frozen soils.
Accelerated polar warming will cause loss of ice and a consequent darkening of the surface, leading to more heat absorption and faster warming. Atmospheric concentrations of aerosols (tiny particulates) are likely to decrease in future as nations improve air quality, leading to accelerated warming as the cooling effect of aerosols is reduced.
Oceans are becoming more acidic as a consequence of increased CO2 in the atmosphere. When CO2 concentrations exceed levels to be reached by 2035 under business-as-usual emissions scenarios, there will be severe disruptions to marine ecosystems (including the Great Barrier Reef and ocean food chains), which will endanger fisheries and weaken the uptake of CO2 by oceans.
Temperature rises of two to three degrees (or higher) carry a high risk of irreversible decay of the Greenland ice sheet from surface warming alone, leading to a sea level rise of up to about seven metres. Destabilisation of the West Antarctic Ice Sheet would cause a further few metres of sea-level rise.
A climate conference in Copenhagen in March concluded that societies were highly vulnerable to even modest levels of climate change, with poor nations and communities particularly at risk. Temperature rises above two degrees will be very difficult for contemporary societies to cope with and will increase the level of climate disruption through the rest of the century.
All of these concerns are firmly grounded in science. They have led the great majority of climate scientists to conclude (paraphrasing the summary of the Copenhagen conference) that rapid, sustained and effective emissions reductions are required to avoid ‘‘dangerous climate change’’, regardless of how it is defined.
Higher future emissions increase the risk of crossing climate tipping points and they increase the likelihood that the long-term social and economic costs of both adaptation and mitigation will be higher.
This article was written by Michael Raupach and John Church, CSIRO; David Griggs, Amanda Lynch and Neville Nicholls, Monash University; Nathan Bindoff, Antarctic Climate and Ecosystems Co-operative Research Centre; Matthew England and Andy Pitman, University of NSW; Ann Henderson-Sellers and Lesley Hughes, Macquarie University; Ove Hoegh-Guldberg, University of Queensland; Roger Jones, Victoria University; David Karoly, University of Melbourne; and Tony McMichael and Will Steffen, Australian National University.