How to stop global warming, in 7 steps

Global warming is here, it’s man-made, and it will cause serious problems in the years ahead. What’s more, humanity has daillied so long that avoiding the worst impacts will now require extremely sharp emissions cuts — and possibly taking carbon out of the air.

That’s the upshot of a major new synthesis report from the UN’s Intergovernmental Panel on Climate Change (IPCC). It provides a helpful summary of the panel’s previous three big reports on global warming, which dealt with: 1) the physical science of climate change, 2) how bad it could get, and 3) how to stop it.

The new report includes a review of the evidence that carbon dioxide from burning coal, gas, and oil is heating the planet. It notes that some amount of “irreversible” climate disruption is already locked in, but things can also get much, much worse. Additional global warming could wreak havoc across the globe, potentially leading to food shortages, the flooding of major cities, and mass extinctions.

Perhaps the most relevant sections are about how to avoid this fate, something the world’s nations will be discussing over the next year of UN climate talks. To avoid the worst outcomes, the world would need to act immediately and drastically, reducing emissions 41 to 72 percent below 2010 levels by mid-century. We’d then need to keep cutting and possibly be taking carbon-dioxide back out of the atmosphere by 2100.

That won’t be easy. And the task gets all the harder if countries delay action or if they rule out certain controversial technologies, like nuclear power or carbon capture for coal plants. Here are seven key points from the report:

The world’s nations have pledged to prevent global average temperatures from rising more than 2° Celsius (or 3.6° Fahrenheit) above pre-industrial levels. If we go too far above that, the worry goes, we dramatically increase the risks of things like rapid sea-level rise or mass extinctions or severe damage to our farms and crops.

Now, countries like Europe and the United States, have made various pledges to cut their emissions in recent years. But even if those pledges pan out, the IPCC estimates, the world would still be on pace for roughly 3°C of global warming by the end of the century. (There’s a range of possible outcomes, but that’s the central estimate.)

So how can we stay below 2°C of global warming?

The IPCC calculates that annual greenhouse-gas emissions would have to start dropping each year — until they were 41 percent to 72 percent below 2010 levels by mid-century. Then emissions would have to keep falling until humans were hardly putting any extra greenhouse gases by the end of the century. We’d also likely have to pull some carbon-dioxide out of the atmosphere.

See the first row below, labeled “RCP2.6”:

This task sounds extraordinarily difficult — and it is. But the IPCC notes that it becomes even more difficult the longer we put off cutting emissions, because carbon-dioxide and other greenhouse gases will keep piling up in the atmosphere in the meantime, and the cuts needed to stay below the limit become more severe.

In fact, if annual emissions in 2030 are still above today’s levels, it becomes all but impossible to stay below that 2°C limit. (And even 3°C would be difficult to avoid.)

So how do we cut emissions that sharply? First, the IPCC says that the world would have to triple or even quadruple the share of clean energy that it uses by 2050 — and keep scaling it up thereafter. Second, we’d have to get dramatically more efficient at using energy in our homes, buildings, and cars.

Right now, about 13 percent of the world’s energy is “low-carbon” — a little bit of wind and solar power, some nuclear power plants, a bunch of hydroelectric dams. Those technologies would need to continue to improve and expand dramatically.

That means two things. First, it’s tough to rule out any particular technologies. For instance, some environmentalists are opposed to nuclear power. But the IPCC estimates that the task of cutting emissions becomes between 4 and 18 percent more expensive if nations shuttered all their nuclear plants. Likewise, the technology to capture carbon emissions from coal plants and bury it underground is still in its infancy. But if that technology proves unworkable or limited, the IPCC estimates, then the task of cutting emissions becomes twice as expensive:

(It’s worth noting that there are factors pushing the other way, too: Solar power has become much, much cheaper since the IPCC drew up its assessment, which makes the overall task of cutting emissions a bit easier.)

Second, the IPCC notes that investment in fossil fuels — coal, oil, and natural gas — would have to decrease by 20 percent in the next few decades. After all, if renewable energy ramps up, but conventional coal expands even faster, emissions would rise, not fall.

Is this all doable? The IPCC report suggests that it’s at least technologically feasible. Whether it’s politically realistic is another matter. The report notes that countries could start taxing carbon emissions as way of pushing private companies to redirect their investments. So far, however, those policies have been slow to catch on — in the United States, a carbon tax is a non-starter in Congress.

Back in its 2007 report on preventing climate change, the IPCC suggested that the world’s emissions would have to peak in 2015 if we wanted to prevent 2°C or more of global warming.

That’s obviously not going to happen — 2015 is next year, and emissions are expected to keep rising. So why does the IPCC think we still have a chance this time around?

The panel is putting its hopes in technologies that allow us to pull carbon out of the atmosphere toward the end of the century. What if, for instance, we grew trees that sucked carbon dioxide out of the atmosphere. Then we burned those trees for fuel. But instead of letting the carbon dioxide from those trees go back into the atmosphere when we burned it, we captured the emissions and pumped them underground? Voilà: That whole process would, in theory, be “carbon-negative.”

The problem? The IPCC concedes that the availability of these techniques is “uncertain” and the technology is currently “limited.” So the panel is putting a lot of hope in an unproven concept to help limit global warming and stay below the 2°C target.

The IPCC estimates that staying below the 2°C target will likely cost us. After all, we’re giving up cheaper fossil fuels and replacing them with pricier electric cars and solar panels and nuclear plants.

Economic modeling suggests that this would shave 0.06 percentage points off global economic growth each year. So instead of growing by, say, 3 percent per year we’d be growing by 2.94 percent. The world would still get richer over time, but at a somewhat slower rate. By century’s end, a massive clean-energy push would have cost between 3 and 11 percent of global income.

That sounds manageable, though it’s also assuming all the necessary technologies work out and that countries start cutting immediately. It also assumes that countries adopt the most cost-effective emissions policies possible — which is far from a given.

One question, meanwhile, is how this compares to the costs of not doing anything. The IPCC notes that it’s difficult to assess the costs of unchecked global warming. More extreme weather and higher sea levels and crop failures were all likely to be quite damaging — but there wasn’t enough research to put a precise dollar figure on it. (The panel suggested that just 2.5°C of warming would cost between 0.2 and 2 percent of annual income in 2100, though it noted that this was likely to be a low-ball figure.)

So a lot depends on how much risk we’re willing to take on. If we pay less for cutting emissions, we’ll likely pay more in damages from higher temperatures — and vice versa. In his recent book The Climate Casino, Yale economist William Nordhaus suggested that the costs and benefits were likely to balance out at around 2.5°C of global warming. But others have come up with higher and lower targets to aim for.

(By the way, there’s a good argument that it’s impossible to get precise cost-estimates for any of this. David Roberts has made that argument at length for Grist. But the broad conceptual points — that it costs more if we delay cutting emissions, and that higher temperatures typically mean higher costs — tend to hold regardless of exact numbers.)

The IPCC notes that all the world’s major nations would have to work together to halt global warming. That’s because additional carbon-dioxide in the atmosphere helps heat up the planet no matter who emits it.

So it’s not like Europe can cut all of its emissions and the problem is solved. Everyone else — China, India, the United States, Japan — would have to reduce their greenhouse gases, too.

And that raises thorny questions. How do the different nations divvy up the necessary cuts? Should wealthier regions like Europe and the United States cut more, since they’re responsible for most of the man-made greenhouse gases that have already been put in the atmosphere? Should they pay poorer countries to help cut emissions? These sorts of questions have often bogged down UN climate talks and led to stalemates.

Even if the world cuts emissions drastically and stays below 2°C of warming, the IPCC notes, we’ve already locked in some amount of “irreversible” climate change, whose effects will “continue for centuries.” That will mean changes in sea levels, rainfall patterns, extreme weather, and so on. And countries all over will have to adapt. Some examples:

-- Africa faces an increased risk of crop failure due to increased heat and drought. Countries can partly offset these risks through things like better irrigation practices, more loans for small farmers, providing access to fertilizer and better farming practices, and creating “early-warning systems” against drought.

-- Asia needs to worry about increased flooding from heavy storms and tropical cyclones, among other things. Adaptation might involve early-warning systems and stricter building codes so that homes can withstand flooding.

-- North America will face increased wildfires and deadly heat waves. Possible adaptations include providing “cooling centers” for people who don’t have air conditioning during heat waves. Governments could also stop subsidizing people who live in wildfire-prone areas.

-- Two degrees: How the world failed on climate change

-- Here was my rundown of the initial IPCC report in 2013, on the science of global warming itself. Here’s a summary of the second IPCC report on the impacts of global warming.