There are plenty of studies on how bad climate change could get in the decades ahead. But how do we actually stop global warming — or at least slow it down?
That's the subject of a big new report from the UN's Intergovernmental Panel on Climate Change (IPCC) published Sunday. The topic this time is "mitigation" — how humanity could stop producing so many of the greenhouse gases that are warming the planet. (This is the third IPCC report over the past year — the first two dealt with climate science and climate impacts.)
The upshot: Global greenhouse-gas emissions from fossil fuels and other sources are rising rapidly — putting the world on pace for significant temperature increases by century's end. To meet its climate goals, the world would have to act quickly, cutting emissions until they were 41 to 72 percent below today's levels by mid-century. That won't be easy. And the task gets much harder if we rule out any technologies, like nuclear power or carbon capture for coal plants.
Here are six key points:
1) Right now, the world is failing badly at its climate goals
Many of 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 get too far above that, the worry goes, then we increase the risks of things like rapid sea-level rise or mass extinctions or severe damage to our farms and crops.
On our current course, it's unlikely that we'll stay below that 2°C limit. Global average temperatures have already risen 0.8°C over the past century, as humans have burned fossil fuels and cleared forests and put more heat-trapping greenhouse gases in the atmosphere.
And, the IPCC notes, yearly greenhouse-gas emissions have kept rising fast in recent decades (see chart). If this keeps up, we're likely on pace for between 3.7°C and 4.8°C rise in average temperature sby the end of the century. The World Bank, for one, thinks that would be a disaster — because "there is no certainty that adaptation to a 4°C world is possible.".
Now, some countries — like Europe and the United States — have made various pledges to cut their emissions in recent years. These pledges might not pan out. But even if they did, 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.)
2) Meeting those climate goals would require sharp emissions cuts — and soon
So how do 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.
That all sounds 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, the report notes, it becomes nearly impossible to stay below that 2°C limit.
3) Cutting emissions will require a huge technological push
So how do we cut emissions that sharply? First, the IPCC says that the world would have to triple or quadruple the share of clean energy that it uses. Second, we'd have to get dramatically more efficient at using energy in our homes, buildings, and cars.
Right now, about 17 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. Countries would have to ramp those sorts of technologies up dramatically — tripling or quadrupling their share.
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 7 percent more expensive if we shuttered all our 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, then the task of cutting emissions becomes twice as expensive.
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 solar power 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.
4) We'll also need to pull carbon out of the atmosphere
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 buried 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.
5) Cutting emissions will cost us — but so will global warming
The IPCC estimates that staying below the 2°C target will cost us — after all, we're giving up cheaper fossil fuels and replacing them with pricier electric cars and solar panels and nuclear plants.
Doing so, the panel estimates, 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. (Though that's assuming all the necessary energy technologies work out and countries start immediately — if they don't, the costs will rise.)
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, the IPCC says.
One big question, of course, is how this compares to the costs of not doing anything and letting the planet heat up. In its previous report on climate impacts, the IPCC noted that it was difficult to assess the costs of unchecked global warming — say, 4°C or more. More extreme weather and higher sea levels and damage to crops and dying coral reefs were all likely to be quite damaging — but there wasn't enough research to put a precise dollar figure on it. (At the lower end, the panel suggested that just 2.5°C of warming would cost between 0.2 and 2 percent of global income in 2100, though it noted that this could 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.
6) Countries would have to start working together for a change
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.
And this report will hardly end the bickering. As Karl Ritter reports for the AP, many countries were already wrangling over the executive summary of the IPCC report, which is written for policymakers. Oil-rich Saudi Arabia, for instance, objected to any specific numbers for emissions cuts. And the United States was pushing for a report that made it clearer that fast-growing countries like China and India needed to take more responsibility than they have so far.
Further reading: 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.