The latest climate report by the Intergovernmental Panel on Climate Change's (IPCC) Working Group 1 was released Friday, Sept. 27, and concludes with higher confidence than ever before that human activity is contributing to climate change.
A press release issued following the publication of the report states that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. The evidence for this has grown, thanks to more and better observations, an improved understanding of the climate system response and improved climate models."
Dr. Chris Forest, associate professor of climate dynamics at the Pennsylvania State University and a lead author on chapter 9, the Evaluation of Climate Models, of the IPCC report said, "The general conclusions of the report are confirming a lot of what we've already known."
Forest attributes the new confirmations to the technological advancements that have been made over the years.
"Since the early 90s, the science has gone through a lot of changes," he said. "The progression of science and our understanding of the climate system and climate models and their projections has gone through a transition. From being atmosphere-only climate models to the newest system of Earth-system models, which incorporate the biogeochemical cycles, ice sheets, changes in vegetation, changes in the carbon cycle, in addition to improvements of all the other representations of clouds, the ocean, sea ice, and so on, and how they all interact together to produce a projection."
Human influence on climate is clear, IPCC report says.
Forest said that initially projections could go up to the year 2100 but are now simulating what could happen up to the year 2300 on a regular basis. Some climate models are being used to project out to the year 3000.
Forest said that a key factor in understanding the way climate models work is to not think of them as predictions but rather as projections.
"Given a possible future scenario of factors that are affecting climate change, we can run a climate model to produce the response of the climate system to those scenarios," he said. "It's not going to be perfectly predicted, but we can see the path that we're on for the next multiple decades, so that's something that's quite straight forward."
Forest said that he believes the science is strong, and that the consistency of climate model results is growing much stronger.
"The goal is to get the probability distributions of climate and the weather events correct, and that's where the models are being tested."
Among the highlights of the report's summary for policy makers, it states that, "It is virtually certain that globally the troposphere has warmed since the mid-20th century."
The report also addresses the Medieval Warming Period, which opposing groups have often used in their arguments that current warming is not the result of increased human activity. The report states with high confidence that while some regions were as warm then as they are now, the warming was not occurring as "coherently across regions as the warming in the late 20th century."
The report states that almost the whole planet has experienced surface warming since 1901 based on multiple, independently-produced data sets.
There are three working groups that release reports for the IPCC. This release came from Working Group 1, which focuses on the basic science of climate change. Next year, groups 2 and 3 will release their reports on climate change's impacts on adaptability and vulnerability, and on mitigation and and policy responses, respectively. New reports are released from these groups on seven-year cycles to provide a periodic assessment of these topics.