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The world’s climate scientists explain how to avoid drastic global warming. It’s not easy.

Humans are warming the planet. Scientists are 95 percent sure of this — as sure as they are that cigarette smoking causes cancer. That's one takeaway from the big new climate report by the Intergovernmental Panel on Climate Change, which began rolling out Friday.

The major report synthesizes the vast array of climate science research out there. Humans have pushed temperatures higher by burning fossil fuels and clearing forests, the report says. If we keep producing heat-trapping greenhouse gases, that will lead to much more ice melt and higher sea levels in the decades ahead.

None of this is too surprising. Climate scientists have known that humans are warming the planet for many years — their confidence has gone up, but the conclusion has stayed the same.

Even so, it's worth reading through the summary of the IPCC report to see what else the climate panel has to say. There are some new details here, including the critical point that humans will have to keep the vast majority of their fossil-fuel reserves in the ground if we want to keep global warming below 2ºC, the level deemed "dangerous" by world leaders.

1) Humans are in control of how much the planet will heat up in the decades ahead. We can choose 1ºC (or less) of global warming. Or we can choose a drastic 4ºC.

But human influences on the climate are becoming increasingly dominant. "By the mid-21st century, the magnitudes of the projected changes are substantially affected by the choice of emissions scenario," the IPCC report says. In other words, how hot the Earth gets will depend on how much carbon-dioxide we put into the atmosphere.

In the chart above, the report shows how hot the Earth is likely to get under different emissions scenarios. If we managed to cut global emissions drastically — that's the blue line — we have a decent shot at keeping global warming below 1ºC. But if emissions keep rising unchecked — the red line — then temperatures could rise around 4ºC (or 7.2ºF).

2) Humans can only burn about one-sixth of their fossil fuel reserves if they want to keep global warming below 2ºC.

For years now, world leaders have agreed that 2ºC is the level of “dangerous” global warming that humanity should avoid. The new IPCC report says we have to keep the vast majority of our oil, gas, and coal in the ground if we want to reach that goal.

The math is pretty straightforward: Humans have already emitted about 531 billion tons of carbon since the Industrial Revolution, by clearing forests and burning fossil fuels. And, scientists calculate, we can only emit another 469 billion tons or so if we want a good shot at keeping global warming below 2ºC. We're currently on pace to do that by 2040.

So how can we stay below 2ºC? Currently, the world has an estimated 3 trillion tons or so of carbon locked away in known oil, gas, and coal reserves. So we would only be able to burn about one-sixth of our remaining fossil fuels. The rest would have to stay in the ground. That's a daunting prospect.

(For those interested, Kelly Levin of the World Resources Institute has much, much more detail on these numbers, and many thanks to her for these calculations.)

3) The oceans will keep rising even if humans stop emitting carbon tomorrow. But we can choose between medium sea-level rise or drastic sea-level rise.

We've already warmed the planet enough to heat and expand the oceans and lock in some melting of land ice. So even if we do curtail emissions dramatically — again, that's the blue line — we can expect some sea-level rise in the decades ahead, possibly half a meter by the end of the century, and continuing thereafter.

But if we keep emitting greenhouse gases at our current pace, then sea-level rise gets even more drastic. The IPCC is projecting up to a meter of sea-level rise by century's end if emissions stay unchecked — that's the red line. Oceans would continue to rise for centuries thereafter.

By the way, the report points out that some models project even higher sea-level rise than one meter by centuries end. These are so-called "semi-empirical" models that have had some success, but don't actually model physical processes. The IPCC doesn't rule out these higher projections, but it can't assess their reliability.

4) Using geoengineering to cool the planet is fraught with risks.

In recent years, some scientists have despaired of the idea that humans will ever cut their greenhouse-gas emissions and have looked to "geoengineering" schemes that could artificially cool the planet. One such idea is to sprinkle sulfate particles into the atmosphere to reflect sunlight. (Volcanic eruptions can lower global temperatures by a similar process.)

The IPCC, however, thinks that this idea does carry some risks, like mucking with global patterns of rainfall. If, for some reason, the geoengineering process was ever halted, temperatures would rise very rapidly:

Methods that aim to deliberately alter the climate system to counter climate change, termed geoengineering, have been proposed. Limited evidence precludes a comprehensive quantitative assessment of both Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) and their impact on the climate system. CDR methods have biogeochemical and technological limitations to their potential on a global scale. There is insufficient knowledge to quantify how much CO2 emissions could be partially offset by CDR on a century timescale.
Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing. CDR and SRM methods carry side effects and long-term consequences on a global scale.

So even a last-resort fix to climate change might have unintended consequences.

* Here's a good primer on what the IPCC is and what it's releasing. Note that only the "summary for policymakers" is coming out today (here), with the full 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.

Further reading: 

--The science of global warming has changed a lot in 25 years. The basic conclusions haven’t.

-- My colleague Jason Samenow takes a look at the IPCC's gamble on its computer models, which have been out of sync with temperature observations in recent years as warming rates have slowed. I wrote about a similar topic here (see point #3, especially).