The biggest problem in the battle between the 97% of scientists who say humans are the main driver of rising temperatures and the sceptics has been certainty. Scientists are loathe to say something is a fact because they are constantly contesting their hypotheses – this is how science works. But when it comes to anthropogenic or human-driven temperature increases, they are as certain as can be.
The body that collates the science around climate and temperature changes is the Intergovernmental Panel on Climate Change (IPCC). This is a United Nations body and released its last tome in 2007. The next one is due to be released at the end of September.
The IPCC's previous report said it was "very likely" anthropogenic greenhouse gas emissions had contributed to more than half of all global warming since the middle of the previous century. This means more than 90% certainty and this figure is expected to increase in the forthcoming report.
But this is still not certainty, so scientists are constantly trying to find "fingerprints" of human influences on the world's climate system. If they can say definitively that human activity led to the temperature increase then certainty would be achieved.
This is what a new report, Human and Natural Influences on the Changing Structure of the Atmosphere, has done. Published in peer-reviewed journal National Academy of Sciences, it concludes: "Our results provide clear evidence for a discernible human influence on the thermal structure of the atmosphere."
The method the research team used was to create models of what the earth’s natural systems would be doing without human interference – a "world without us". They then created models where the only impact stemmed from humans, with the natural systems removed. Their last models worked out how much the world changed naturally with long-term events such as El Nino.
This allowed the team to find changes in the climate that can only have been caused by humans, because they come up in the “human only” scenarios that they created.
The clear human "fingerprints" that they found led to occurences such as the constant warming of the troposphere – the lowest part of the atmosphere, where all life exists. The results "underscore the dominant role human activities have played in recent climate change", the research concluded.
The research tackled the current slowing down of long-term temperature increases. Radiation from volcanic aerosols in the upper atmosphere lead to a slowing down of the amount of solar radiation reaching the earth's surface. This cools the lower levels of the atmosphere.
It will take more than a decade for the normal, rapid temperature increases to come back in full force because the oceans trap so much heat and only slowly change, the report says.
The findings are likely to be confirmed with the release of the next IPCC report. This Fifth Assessment Report is compiled by 831 experts in 85 countries and is seen as the last word on climate change.