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The Known Knowns of Climate Change

POTSDAM – The philosopher Daniel Dennett once compared science to the construction of a huge pyramid. Its base comprises the mass of well-established knowledge – no longer controversial and seldom discussed outside academia. More recent research is piled toward the top of the pyramid, where most public debate takes place. It is an apt metaphor for climate-change research, and one worth bearing in mind with the publication of the latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC).

The IPCC’s fifth report, the product of several years of work by hundreds of climate scientists around the world, reviews our established understanding of climate change and explains more recent findings. The media understandably tend to focus on the latter – like the much higher sea-level rise predictions compared to the previous IPCC report of 2007. But let us step back from the news cycle to look at the solid knowledge base of our pyramid.

Climate research dates back at least two centuries, to Joseph Fourier’s discovery of the effects of greenhouse gases on planetary climates; in 1859, John Tyndall demonstrated in his laboratory which gases cause this effect. Detailed radiation measurements on the ground and from satellites have since proved the greenhouse effect’s existence.

We also know beyond doubt that emissions from human activities have substantially increased the amount of greenhouse gases (especially carbon dioxide) in our atmosphere. When the first IPCC report was published in 1990, the concentration of CO2 in the atmosphere had reached 354 parts per million (up from the preindustrial baseline of 280 ppm). This year, the concentration of atmospheric CO2 crossed the 400 ppm line for the first time. CO2 levels are already far higher than they have been in a million years, as ancient air bubbles trapped in the Antarctic ice show.

We know that the amount of greenhouse gases is rising due to our emissions, and we know that this is causing warming. But how much? The most telling number here is the “climate sensitivity” – that is, the degree of global warming caused by a doubling of atmospheric CO2. IPCC reports have consistently given a range of 1.5-4.5ºC (with a minor exception in the fourth report, which gave a range of 2.0-4.5ºC). Natural climate changes in the past – for example the great ice ages – attest to the sensitivity of our planet’s climate to disturbances, and analyzing them is one method used by scientists to estimate this sensitivity.

An extraordinary, if underappreciated, feature of the IPCC’s reports is that, though many different scientists have worked on them over the past 23 years, the fundamental conclusions have not changed. This reflects an overwhelming consensus among scientists from around the world. Polls of climate researchers, as well as analysis of thousands of scientific publications, consistently show a 97-98% consensus that human-caused emissions are causing global warming.

Yet these conclusions need to be reaffirmed in the face of efforts by well-funded special-interest groups to sow doubt among the public. Indeed, these efforts have been so successful that few members of the public are aware of the scientific consensus on the fundamentals of climate change. Many believe that there is controversy where there simply is not.

The past can serve as a guide to the consequences of the warming that we are causing. Scientists studying paleoclimate – the climates of the ancient past – have documented the massive impact of earlier climatic changes. At the end of the last Ice Age, for example, global temperature rose by about 5ºC over a period of 5,000 years. This was enough to transform the Earth’s vegetation cover, melt two-thirds of the continental ice masses, and raise sea levels by more than a hundred meters. Slowly but surely, sea levels are inching up once again. A key conclusion of the new IPCC report is that sea-level rise has accelerated.

But, before millions of people are submerged, many will be struck by extreme weather events. Record-breaking hot months now occur five times more frequently than they would in a stable, unchanging climate; these heat waves cause droughts, wild fires, poor harvests, and, inevitably, loss of life.

The latest IPCC report describes our current predicament with disturbing clarity: global temperatures are climbing, mountain glaciers and polar ice caps are melting, sea levels are rising, and extreme weather events are becoming more frequent and more severe.

The details near the top of the knowledge pyramid can and should be intensely debated. But our solid understanding of the fundamentals of global warming – the base of our knowledge of climate science – should provide reason enough to press on with the implementation of carbon-free energy technologies. With a rapid reduction in emissions, it is still possible to keep warming within safe bounds (estimated at below 2ºC); but the task is becoming increasingly difficult. Failure to act quickly and globally will leave our children and grandchildren struggling to adapt to rapidly rising seas and devastating weather.