The Intergovernmental Panel on Climate Change (IPCC) released its fifth report (AR5) on climate change on Sep 27, 2013. The full report, which was released on Sep 30, is over 2200 pages long, but the “Summary for Policymakers” is 36 pages long. Every five years or so, the IPCC publishes its assessment of climate science and the study of anthropogenic climate change. Said assessment is based on the results of a years-long, rigorous review of peer-reviewed papers by scientists from all over the world.
The assessment report includes the work of 209 lead authors, over 600 contributing authors, and 50 review editors from 39 countries. The paper cited over 9,200 scientific publications and used over two million gigabytes of data from climate models. The publications examined include studies on such phenomena as rising sea levels, atmospheric changes, and melting ice sheets.
A big change from the 2007 report is scientists’ increased confidence that humans are causing climate change. In 2007, the IPCC had 90% confidence (“very likely”) that human-caused greenhouse gas emissions were the primary cause of climate change– and had been since the mid-20th century. In the new report, the IPCC has 95% confidence (“extremely likely”) that human activities are the main cause. Scientists’ confidence that humans were causing climate change has risen with each report– and the first such report was written in 1990.
The IPCC is also very certain that melting ice, rising sea levels, and various forms of extreme weather are all linked to global warming. In fact, the 2013 report has increased its projections for the future rates of both Arctic sea ice melt and sea level rise to correct for several overly conservative estimates made in the 2007 report. In the 2007 report, scientists predicted that sea levels would rise between 7 and 23 inches by 2100– a projection that was criticized as too conservative, given the pace of melting in Greenland, the Poles, and other places. The new report predicts that sea levels will rise by almost three feet if greenhouse gas emissions aren’t curbed. This could have dire consequences for such major cities as Miami, New Orleans, London, and Shanghai. The Greenland and Antarctic ice sheets have been losing ice for the past two decades and will continue to do so.
Another big change is that atmospheric carbon dioxide is higher than it has been in over 800,000 years. Methane and nitrous oxide have also increased by similar proportions. To make matters worse, much of the change is recent. Carbon dioxide concentrations have increased by 40% since pre-industrial times– roughly 200 years ago. 30% of the carbon dioxide has been absorbed by the oceans, making them more acidic.
The IPCC report also explains the so-called “slowdown” or “hiatus,” which deniers claim means that global warming is slowing down. The scientists note that while temperatures over the last 15 years did not increase as much as originally predicted, the past decade was still the warmest on record. Furthermore, the overall trend is still up: Temperatures are still increasing. The slowdown itself can be explained by natural causes like El Nino events and warming oceans, which are trapping greenhouse gases. Scientists warn that if the oceans start taking up less heat or gases, the rate of global warming will increase again.
The only way to limit climate change is to curb greenhouse gas emissions. However, there is already so much carbon dioxide in the atmosphere that warming will continue for centuries even if humans completely stopped emitting greenhouse gases. But, while some warming is inevitable, the question is how much. The IPCC report predicts global temperature increases between 1.5° C and 4.5° C (2.7° F and 8.1° F). To avoid the worst consequences of climate change, humans need to keep the temperature increases below 2°C (3.6 F).
The IPCC’s report on the science of climate change is only the first part. The second part, which describes the impacts of climate change, will be released in March, 2014. The third part, about mitigation, will be released in April, and a “synthesis report” will be released in October.