Ars asked for your questions about the recently released final draft of the first section of the new Intergovernmental Panel on Climate Change (IPCC) report, and you delivered. A number of the questions were about the climate change impacts and mitigation sections that have not yet been released, but we’ll certainly keep those topics in mind when that information comes out next spring. There are also a few questions related to complex topics that we plan to cover in more detail soon.

All that said, here are some of the biggest reoccurring questions we were able to answer.

How are the IPCC reports written? Do governments get to edit it?

The process is laid out nicely on the IPCC website. A number of scientists are chosen from nominations to be lead authors for different chapters of the report. For the physical science section, for example, 259 lead authors were chosen for its 14 chapters—each summarizing existing research on a different topic. The lead authors take on this project as unpaid volunteers, receiving nothing but some time from their employers to fit the work into their schedules.

Those lead authors prepare a couple rounds of drafts that go out to reviewers who submit a dizzying tally of comments. The reviewers include officials from the participating governments (who look at the second-to-last draft and mainly ask for clarification or for additional topics to be covered) as well as hundreds of experts (who review each draft). The lead authors attempt to address all the comments, but they’re the only ones actually writing the report.

The Summary for Policymakers, a brief rundown of the 2,000+ page report’s conclusions aimed at government officials, works a little differently. After going through the same process, the Summary for Policymakers is subjected to line-by-line scrutiny by the government representatives who gather to finalize the report. Edits to the wording of the summary are discussed (again, mostly for clarity) during this marathon committee session, which the lead authors participate in as well.

Is the report out of date by the time it’s completed?

In order to prevent the huge task of writing the report from being Sisyphean, there’s a cutoff date for studies to be included. For the physical science section, papers had to be accepted for publication by March 15, 2013. While that made for a bit of a mad rush of researchers trying to sneak papers in under the deadline, it also meant that some interesting recent papers were not included.

However, nothing so revolutionary as to render the report obsolete came out. Given the media fascination with the surface warming trend of the past few years, several relevant papers published in the last year might have added to the information in the report, but so it goes.

What are the biggest changes over previous reports, and what new research was behind them?

We touched on this when the final draft of the physical sciences section was released, but there were a couple notable changes. The report’s summary statement on the human contribution to the warming of recent decades stepped up in confidence. While the last report (from 2007) said that “most” of that warming was “very likely” (IPCC code for more than or equal to 90 percent confidence), the new report states that “It is extremely likely [95 percent confidence] that human influence has been the dominant cause of the observed warming since the mid-20th century.” There was no breakthrough discovery that precipitated this; the accumulating evidence simply becomes clearer and clearer over time.

The projections for sea level rise over the 21st century also changed. The last report’s projections were quite conservative, leaving out poorly quantified aspects of the way ice sheets will behave as the climate warms. While that remains pretty uncertain, the science improved enough that these effects were included—still conservatively. For example, the oft-used middle-of-the-road emissions scenario A1B came with a sea level rise estimate of 0.21 to 0.48 meters (0.68 to 1.5 feet) by the 2090s in the last report. While new emissions scenarios are being used this time around, A1B was revisited, with the estimate rising to about 0.35 to 0.68 meters (1.14 to 2.23 feet) averaged over the last two decades of the century. (That’s not strictly an apples-to-apples comparison, but it gives you some idea.)

Leaving out the optimistic scenario in which we actually start removing CO 2 from the atmosphere, the projections now stretch from 0.35 to 0.98 meters (up to 3.2 feet) by 2100.

How have the predictions from earlier reports fared?

Surface temperatures get the most attention, and the new report includes a (busy) chart showing projections from the previous four reports. Several data sets of observed global surface temperature are shown with the blue, green, and dark yellow lines and blocks. (The lines are the smoothed version of the points represented by the blocks.)

Projections from the first three reports (FAR stands for First Assessment Report, etc.) are shown with the yellow, green, and light blue wedges. Individual model simulations from three emissions scenarios in the fourth report in 2007 comprise the spaghetti jumble of orange and purple lines. The bars on the right show the range of projected temperatures in 2035 unobstructed by overlapping lines.

Of course, that’s not the only projection. The chart below, laid out similarly to the one above, shows sea level rise. Note that the fourth report didn’t give a comparable range for sea level rise, so a published study using the emissions scenarios from the report is shown instead.

We could go deeper if we wanted to, since many of the impacts of greenhouse-gas-induced warming described in the report could be considered predictions of sorts. That would include things like the pattern of temperature change in the atmosphere, the lack of natural warming factors that could account for the observations, and even the ocean acidification caused by rising atmospheric CO 2 .

Why did the estimate of equilibrium climate sensitivity change? Does that affect the projections?

Equilibrium climate sensitivity is a specific way to describe the amount of warming you would get in the long term if you doubled the concentration of CO 2 in the atmosphere and waited a very long time for the climate to stabilize. In the 2007 report, the likely range was given as 2.0 to 4.5 degrees Celsius for a doubling of CO 2 , with a best estimate of 3.0 degrees Celsius. That estimate represented a narrowing of the previous report’s range of 1.5 to 4.5 degrees Celsius. In the new report, the likely range has returned to the older numbers, and a best estimate is not given.

There are a number of ways of estimating this value. Just for kicks, here’s a figure showing the numerous estimates summarized by this likely range of 1.5 to 4.5 degrees Celsius.

Picking a likely range doesn’t affect the equilibrium climate sensitivity of the various climate models, which is determined by all the interactions within the model itself. So no, this didn’t have any impact on the projections.