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). The topic this time is “mitigation” — how humanity could stop producing so many of the greenhouse gases.
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.
Turning wind turbines of an onshore wind farm standing on a field on March 15, close to Stoessen, Germany. Renewable energy sources like wind power are fed into the power grid. Thomas Trutschel/Photothek via Getty Images
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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?
“The world would have to cut emissions 41% to 72% by 2050 — and start soon”
That’s the subject of a big new report from the UN’s Intergovernmental Panel on Climate Change (IPCC). The topic this time is “mitigation” — how humanity could stop producing so many of the greenhouse gases that are warming the planet.
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.
“Even if nations follow through on their climate pledges, we’re still on pace for 3°C of global warming”
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 of global warming by the end of the century. The World Bank, for one, thinks this much warming would be extremely challenging to adapt to.
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?
“if emissions in 2030 are still above today’s levels, that 2°C goal becomes impossible”
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.
“if we rule out technologies like nuclear power, the task becomes much harder”
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?
“it’s not clear that we’ll have the technology to pull carbon out of the atmosphere ”
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 be costly — but so will global warming
The IPCC estimates that staying below the 2°C target will be expensive — after all, we’re giving up cheaper fossil fuels and replacing them with costlier electric cars and solar panels and nuclear plants.
“Cutting emissions would cost us — but that needs to be compared to the costs of unchecked warming”
All told, the report predicts, the world will still be richer in 2050 or 2100 than it is today. But ramping up clean energy and reducing greenhouse-gas emissions enough to stay below the 2°C limit will cost us around 5 percent of our income by the end of the century.
How does that compare to the cost of global warming? This is harder to estimate. In its previous report on climate impacts, the IPCC noted that it was difficult to assess the costs of unchecked 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 costly — but there were too many unknowns 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 0.5 and 2 percent of global income — with higher levels quite plausible.)
So a lot depends on how much risk we want to take on. If we pay less for cutting emissions, we risk paying 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 align at around 2.5°C of global warming. But others have come up with higher and lower targets.
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.
How much could global warming cost the economy?
It all depends. Mainstream economic models tend to agree that global warming will hurt economic growth over the long run — particularly if we get more than 1° Celsius (or 1.8° Fahrenheit) of warming. That’s because the damage from things like sea-level rise will outweigh the benefits of things like fewer cold-related deaths.
But different models can give different estimates. The Intergovernmental Panel on Climate Change (IPCC) suggested that 2.5°C of global warming could cost the world between 0.2 percent and 2 percent of its income by century’s end. But the panel wasn’t very confident in those numbers, and noted that the actual damage could be higher.
For one, the IPCC noted, economic estimates of global warming “depend on a large number of assumptions, many of which are disputable.” Key questions include the effects of global warming on the food supply or how well we’ll adapt. For another, economic models have a hard time quantifying the value of, say, species that could go extinct. And there aren’t very good estimates for very high global warming — say, 4°C.
A related question is whether it’s cost-effective to stop global warming. The answer here depends on lots of things, like the costs of reducing greenhouse-gas emissions, how we value future generations, and whether we want to risk extreme outlier scenarios. Yale economist William Nordhaus has argued that stabilizing global warming at around 2.5°C is likely to be the most cost-effective course of action, but others have come up with higher and lower numbers.
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