IPCC Still Really, Really Sure We're Causing Climate Change

The latest assessment from the Intergovernmental Panel on Climate Change, released in full online today, details what we can and cannot predict about global warming.

Changes In Temperature, Historical Vs. Future

IPCC WGI AR5, SPM-1

Today, the UN's Intergovernmental Panel on Climate Change finally released the final version of the first segment of their Fifth Assessment Report, a comprehensive document detailing essentially everything scientists have to say about climate change—or rather, everything that hundreds of scientists and reviewers can agree upon. This report comes out of Working Group I; there will be two more working groups that will issue reports on climate change impacts and vulnerabilities, as well as possible solutions, in March and October of next year, respectively.

The actual conclusions of this report don't diverge a whole lot from the previous assessments. It "represented more of a refinement," according to Graeme Stephens, Director of the Center for Climate Sciences at NASA's Jet Propulsion Laboratory. Stephens was a lead author, a contributing author and an expert reviewer on three different chapters within the assessment. "The conclusions fundamentally are, the planet is warming," he tells Popular Science. Many of the changes between this assessment and the last one, published in 2007, have to do with refining the models used to predict climate change outcomes.

What the panel found

As the IPCC has declared before, in previous assessments and in previous drafts of this assessment, human activity is contributing to climate change. Like, really. In the press release accompanying last week's Summary For Policymakers, the IPCC called global warming "unequivocal" and stated "Human influence on the climate system is clear." The Summary for Policymakers, a condensed version of the complete 2,000-something page report, declared that the period 2016–2035 will likely be 0.3 degrees Celsius to 0.7 degrees Celsius warmer than 1986–2005 was, in terms of mean global surface temperatures.

'The heat is on. Now we must act.'

"It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales as global mean temperatures increase," the report found. "It is very likely that heat waves will occur with a higher frequency and duration." (It's worth noting that the IPCC doesn't do its own research, but rather assesses and reviews published science. This report, years in the making, looked at more than 9,000 peer-reviewed studies and involved more than 600 contributing authors from 39 countries, who took into account more than 50,000 comments on the report's drafts.)

Changes In Surface Temperature Since 1901

IPCC WGI AR5, SPM-1

"The heat is on. Now we must act," UN Secretary-General Ban Ki-moon said during a press conference Friday, demonstrating that no climate situation is too grave for punning. But grave it is. The report indicated that even acting now wouldn't be enough to stop the effects of climate change. "Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped," Friday's summary stated.

Other noteworthy messages it sent to politicians: "It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic [human-caused] increase in greenhouse gas concentrations and other anthropogenic forcings together," according to the summary. In other words, the majority of the increase in worldwide surface temperatures has been the result of human activity.

Things that it's very likely humans influenced: warming oceans, lost Arctic sea ice, the global rise in sea levels and changes in ocean salt levels. It's likely that we also contributed to retreating glaciers and increases in the moisture content of the atmosphere and changes in precipitation patterns. What exactly does that mean? Use this chart:

Uncertainty Scale

IPCC

The level of confidence in a certain prediction usually has to do with how well the climate change models the scientists use can predict our current climate. "We spend a lot of time using current observations from satellites and other sources to compare the models to today, to essentially score them," Duane Waliser, another JPL scientist who served as an expert reviewer on the chapter evaluating climate models, tells Popular Science. So if a certain aspect of the model doesn't match up to the current observations, scientists are less confident that it's an accurate representation of what will happen. "The more places you can compare to an observation, the more likely the whole thing is working better," he says.

More accurate models

Over a long period of time, such as the six years since the last assessment, "almost every component of the climate model gets better in some ways," Waliser says. "It’s just a very subtle but monotonic increase. Every year it gets better."

For example, the models can now be run at a higher resolution. Whereas previously, climate change modeling divided the world into grids about 150 kilometers to 200 kilometers across, now scientists can model a region of about 100 kilometers to 150 kilometers across. It's still not quite refined enough to represent individual thunderstorms or hurricanes, but it does give us more details to represent what's going on in different parts of the world.

The models are 'quite realistic in a global sense, but locally we have a real problem.'

There are also aspects of the models that are more interactive—meaning they can take into account how one aspect of climate change affects another over time. "In the predictions that would have been represented like 10 years ago, what would happen is we would specify the amount of carbon dioxide in the atmosphere and presume we know the growth rate in the next century, and then presume the physical parts of the climate system are just reacting to that. But in nature if it rains more in one place than it used to—or less—the vegetation will change." And how many plants growing in an area affects the carbon cycle and thus, how much carbon dioxide is released into the atmosphere. An interactive model takes that feedback loop into account.

However, there are still things we're not able to interactively model, such as the way ice sheets in places like Greenland and Antarctica melt. The melting ice might change the brightness of an area, which could affect the way it warms and the sea level rise.

Change In Precipitation

IPCC WGI AR5, SPM-1

The models also struggle to predict changes on a smaller scale. They can't really tell us whether California, for example, will have an increase or decrease in rainfall. "One of the areas that they’re really deficient in is in terms of predicting local-scale changes in precipitation," Stephens explains. The models are "quite realistic in a global sense, but locally we have a real problem."

One thing that's helping improve models: getting scientists to use data gathered by NASA satellites for their climate models. "It was evident to us that satellite observations are fairly pervasive, and possibly underutilized for the purposes of model evaluation," Waliser says. As an institution, JPL, along with NASA and the Department of Energy, has tried to make certain satellites more readily accessible for use in climate models, an effort Waliser says he could see paying off in this assessment. "There was more utilization of these measurements. That continues to grow."

Waliser found nothing particularly surprising about the projections of global warming in the model. "It’s evolutionary in that way," he says. "I see the models behind the scene continuing to get better."