Yet, in some ways, what's most striking is how little the report alters our broad understanding of global warming.
This is, after all, the fifth assessment report that the IPCC has put out since 1990. And the main message hasn't shifted all that radically over the last 25 years. Yes, there have been new advances in key areas, from climate history to predicting the effects of sea-level rise. But the core point — that humans are heating up the planet significantly — has stayed remarkably consistent over the past quarter-century.
A consistent message on warming
Back in 1990, climatologists had a relatively good grasp of how humans were warming the planet: "Emissions resulting from human activities are substantially increasing the atmospheric concentrations of greenhouse gases," they wrote in the very first IPCC report. "These increases will enhance the greenhouse effect, resulting on average in an increase in global warming."
Over the years, a vast pile of climate research has simply reaffirmed that conclusion. In 2001, the third IPCC report said it was "likely" that most of the rise in surface temperatures over the past half-century was due to human activity. The 2007 report bumped this up to "very likely." And the 2013 report says that it's "extremely likely" that humans are responsible for more than half that warming — a more than 95 percent chance.
That increased confidence represents years of painstaking research, bolstered by dozens of independent lines of evidence, from advanced computer modeling to investigations into Earth's ancient climate history. Scientists are now as certain that humans are responsible for rising temperatures as they are that cigarette smoke causes lung cancer. But they're still basically saying the same things they said in 1990:
"A lot of these things were actually predicted even before 1990," says Spencer Weart, a physics historian and author of The Discovery of Global Warming. "But they weren't taken quite as seriously by the wider scientific community until around 2000. The biggest change came between 1990 and 2001" — as computer models improved rapidly and the field of paleoclimate offered new independent confirmation of various climate science ideas.
It's also notable that the big uncertainties that existed in 1990 still exist today. One crucial question is how much temperatures will rise each time we double the amount of carbon-dioxide in the atmosphere. In 1990, scientists predicted the Earth would warm somewhere between 1.5ºC to 4ºC. This year? The new report says climate sensitivity is likely... between 1.5ºC and 4ºC for every doubling of carbon-dioxide.
Key details have changed
To be sure, there have also been many notable new discoveries. Computer models, for one, have become adept at simulating the climate system. Back in 1990, the IPCC relied on just two relatively crude models. Today, the IPCC can take advantage of 45 different models that incorporate a wide range of features of the Earth's climate, from ocean biology to changes in soil. Their accuracy has improved considerably since 1990. But it's unclear if those models can keep improving significantly.
"We may have reached the limit of what we can see with computer models," says Weart. "At this point, and this was true in 2007 too, the main uncertainty depends on what happens with man-made emissions. That's something models can't tell us."
Climate scientists also have a much better grasp today of how sea levels will rise as the oceans warm and the ice caps melt. In 1990, it was difficult to measure things like the melting of Greenland and Antarctica. And even in its 2007 assessment report, the IPCC could only explain about 60 percent of the rise in sea-level that had already occurred in the previous half-century.
Nowadays, however, climate scientists can account for virtually all of the past sea-level rise, and some are now predicting that ocean levels could rise 1 meter or more by 2100 as the Earth heats up. (For more on the history of sea-level science, see Nicola Jones's retrospective in Nature.)
Climate scientists are also more keenly aware of other changes in the Earth's climate that were less-appreciated in 1990. The first assessment report barely dwelt on the fact that rising carbon emissions would alter the pH levels in the ocean. Today, that's a major area of concern, as scientists realize that rapid ocean acidification could disrupt everything from coral reefs to oyster fisheries.
Is it time for a new IPCC process?
Yet despite these important advances, the big sweeping IPCC reports may well have reached their endpoint. The fifth report, after all, won't drastically alter our conception of global warming. It will fill in vital new details and address some of the controversies du jour, such as the widely noted recent slowdown in the rise of Earth's surface temperatures. But it probably won't change many minds.
That's why many experts have begun urging the IPCC to rethink its whole mission. Do we really need hundreds and thousands of scientists devoting years of their lives to an encyclopedic synthesis that is getting a bit repetitive? Would it be more useful for the IPCC to produce more frequent, nimble reports on important sub-topics, as it did in 2012 when it released a smaller "special report" on extreme weather?
It's a topic the IPCC has even begun asking itself. "What sort or products should the IPCC be producing, over what kind of time scale?" spokesman Jonathan Lynn recently told the Guardian. "Do we need this blockbuster report every six or seven years or do we need more frequent reports?"
* Here's a good primer on what the IPCC is and what it's releasing tomorrow. Note that only the "summary for policymakers" is coming out Friday (here), with the big 2,000-page scientific report on climate science released Monday. In the months to follow, the IPCC will also release separate reports on the human impacts of climate change and a look at the world's energy system.
-- Nature has a great graphic showing how the reports have changed (or not) over time.
-- Global warming appears to have slowed down lately. That's no reason to celebrate.