Climate change is changing. In three days we will find out how much, and how rapidly. The Intergovernmental Panel on Climate Change (IPCC), which is meeting in Yokohama, Japan, will release the second report of its massive assessment on Sunday, March 30 EDT (March 31 in Japan). The report will weigh the impacts of climate change today and into the future, as well as how vulnerable people and places are to those impacts. A draft of the report, leaked last year, indicated that major risks such as drought, flooding, hunger, disease and stunted economic growth would begin sooner than previously estimated.

The IPCC won’t necessarily tell us when, exactly, we will cross the line into major trouble. But I’ll have an answer to that in a moment.

A lot is riding on this meeting because it marks a clear change in strategy for the IPCC. Despite increasingly dire warnings it has issued for 24 years, various polls show that as many as half of Americans still are not sure that the science is certain. So the hundreds of scientists from around the world who are involved in this report will present the ongoing findings in terms of risks, rather than data and error bars. The spread ranges from risks that are highly likely but may have modest implications, to those that may be unlikely but have severe consequences, such as runaway melting of Greenland, which could raise sea levels dramatically.

The report will also present a variety of possible solutions, such as better disaster planning, the breeding of drought-resistant crops and technologies that can save energy.

Talking about risk and how to lessen it is also a strong attempt by the IPCC to put climate-change denial to rest. The IPCC is no longer focusing on defending the science; it is moving on, trying to advise cities, states and countries about the regional risks that will confront them, and what can be done.

Another major report released last week by the American Association for the Advancement of Science upholds this strategy, as well as the expected IPCC findings. AAAS billed the report, called What We Know, as an initiative to increase dialogue on the risks of climate change. Rather than needlessly arguing over the science, "We can debate the policies," said James McCarthy, an oceanographer at Harvard University and co-chairman of the report. "And that's the debate we should be having.”

Policies are needed soon because the new IPCC report is expected to show that climate change impacts are happening on every continent, that people in all nations are vulnerable to extreme climate events, and that if the world does not begin to act soon it will miss the chance to limit global warming to less than 2 degrees Celsius—a goal that leaders of almost 200 countries support.

What IPCC seems unlikely to say, however, is when the world will cross that two-degree line. The AAAS report does not give a date either. Part of the reason is that being so specific requires scientist to settle on just how sensitive the atmosphere is to a rising level of greenhouse gases. This level is usually expressed as a concentration of CO2, which has been mounting steadily for decades. Estimated at roughly 280 parts per million when the Industrial Revolution began, it briefly hit 400 ppm last May—the first time it reached 400 ppm since humans have been on Earth. This year it hit 400 ppm on March 12, and will linger there longer, until plants and trees grow their spring leaves and absorb some of it, bringing the number down into the 390s. The levels will continue to ratchet up annually if nations everywhere do not begin to reduce CO2 emissions.

As CO2 levels rise, so does the average global temperature. It has already risen 0.8 degrees C since pre-industrial times. But when will it cross the 2-degree-C threshold, if nations do not change? The answer comes from Michael Mann, and it’s very soon: 2036.

Mann, a professor of meteorology at Pennsylvania State University, was a key contributor to the third IPCC assessment in 2001 (the latest is the IPCC’s fifth, since 1990). That work resulted in the so-called hockey stick: a curve that shows a global average temperature that is roughly steady for 1,000 years but then turns abruptly upward in recent times. Mann is not involved the current IPCC assessment. But he knows the numbers from the latest science that IPCC is using, so he did some calculations and published them in the current issue of Scientific American.

Mann first settled on the two most likely numbers for the “equilibrium climate sensitivity.” ECS is a common measure of the heating effect of greenhouse gases. He then plugged those ECS values into the so-called energy balance model, which scientists use to investigate possible climate scenarios.

The model showed that if the world continues to burn fossil fuels at the current rate, the planet will cross the dangerous warming threshold of 2 degrees C in 2036, only 22 years from now, when using the higher sensitivity number, which Mann thinks is most realistic. When he entered the lower ECS value—very conservative, in his view—the world crossed the threshold in 2046, just 10 years later (see the graph above). The truly tough news revealed by these and further calculations is that to reliably avoid 2 degrees C of warming, the world should hold CO2 levels below 405 ppm, barely above where they are now.

You can see his numbers, graphs and conclusions. And you can even do the calculations yourself, if you like.

Mann’s projections are probably bolder than what we will hear from the IPCC, because he doesn’t have to get scores of nations to settle on wording for a report. But the IPCC and AAAS have the same message as he does: nations everywhere will have to act fast if they want the planet to stay below the danger threshold. IPCC will reveal its language on Sunday night. Scientific American will be reporting it, so stay tuned.