After several years of work by over 800 scientists the Intergovernmental Panel on Climate Change (IPCC) has presented its fifth assessment report on climate change. According to the report, warming of the climate system is unequivocal and unprecedented
The Key message is unambiguous; human influence has been the dominant cause of the observed warming since the mid-20th century humans cause climate change, and the burning of fossil fuels is the main reason behind a 40 percent increase in carbon-dioxide concentrations since the industrial revolution. Other findings of the latest IPCC report include: global temperatures are likely to rise by 0.3 to 4.8 degrees Celsius by the end of the century; and, sea levels are expected to rise a further 26-82 centimeters by the of the century.
But the gravity of the IPCC report has run into a significant PR headwind. The call for urgent action seems diminished by what seems to be a slowing down or a hiatus in global warming. Scrutinizing the global temperature curves between 1850 and 2012, detractors have chosen to focus on the “wiggles” toward the end of the time series. These “wiggles” have emboldened climate change deniers to claim that there has been a slowing down in global warming, which has lasted 15 years.
A paper published in the current issue of the Nature Climate Change journal nearly all of the one hundred climate-model simulations they examined overestimated global warming over the past 20 years. The discrepancy between observed and simulated warming is more striking over the past 15 years (1998-2012) where the actual average warming trend per decade, 0.05 degrees Celsius, is more than four times smaller than the average simulated trend per decade.
To its credit, the IPCC report recognizes the hiatus and notes that global mean surface temperature exhibits substantial decadal and interannual variability. Authors of the IPCC’s fifth assessment report observe that trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. But skeptics see the hiatus as evidence that the IPCC concerns are overblown. The skeptics have also pointed to the fourth assessment by IPCC in 2007, which exaggerated the rate of melting of glaciers in the Himalayas and overstated risk of floods in the Netherlands.
As people trained in the science we do not know as much as we claim about how our planet’s climate works. Global warming, and especially its effects on our planet, is a lot more complicated than we have assumed. More importantly, because of the complex interactions and feedback there is no simple relationship between greenhouse gas emissions and global warming. This not to say that loading up our atmosphere with water vapor carbon dioxide, methane and nitrous oxide does not trap long wavelength radiation, leading to warming of our planet.
The physical science basis of climate change is not in doubt. And just like previous IPCC reports, the fifth assessment that was released last Friday makes this even clearer. The point I want to make is that the effect of greenhouse gases on the earth’s climate is mediated by complex interactions, producing feedback, which we do not yet fully understand and are unable to model well. Hence, it is possible that our rather simplistic models will often overestimate global warming.
Science in its current state can predict accurately surface temperature increases over the course of the next decade, let alone a century. Global surface temperatures could increase by 0.5 degrees Celsius or by 4.5 degrees Celsius. The point is, the magnitude of warming as well as its impact is surrounded by significant uncertainty and simply beyond the current capability of science to determine with certainty.
Doubt and skepticism is the lifeblood of science, and at the heart of scientific discovery and innovation. But politics, policy and public opinion are predicated on the illusion of the so-called facts. Hence, uncertainty is a social anathema. But uncertainty over the magnitude of global temperature rise and the scale of associated impact must not be taken as the tranquilizing drug for inaction. Uncertainty is an essential characteristic of human induced climate change and hence the raison d'être for mitigating action.
We know enough to act to forestall dangerous warming. The critical response to global warming must be driven by moral accountability to posterity, not narrow national politics and economic calculus.