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We Know Humans Are Causing Global Warming; Here Are Some Things We’re Less Sure About

Here, gleaned from the IPCC’s briefing, are some of the things we still don’t know much about

September 27, 2013

Melting Greenland glaciers will have an effect on the global climate by affecting the strength of ocean circulation patterns. Exactly how much of an effect they’ll have is stlll up in the air. Christine Zenino

This morning in Sweden representatives from the Intergovernmental Panel on Climate Change presented a summary of the current state of scientific knowledge about climate change, a brief version of part of the IPCC’s upcoming full report. Most of the attention is being paid—and rightly so—to the things we know we know for sure: the temperature is rising, the sea level is, too. And we and our carbon emissions are largely to blame.

The IPCC report speaks a language of certainties and uncertainties—what do we think we know? how certain are we about it? The headline news from this new IPCC report is that we’re overwhelmingly certain that people are causing climate change. But what are we less confident about? The short answer is: we’re less sure about what’s happening in places where there’s less data— whether because historically there’s been less funding for science there, as in places outside the northern hemisphere, or less human presence, as in Antarctica.

This doesn’t undermine the IPCC’s claims: these sources of uncertainty were all taken into consideration when the IPCC said that we’re the dominant driver of climate change. Rather, they’re a reminder that though the science of climate change is settled, it isn’t complete. There’s a lot more work for scientists to do, and many open questions—some of them quite large. Answering these questions will do a great deal to help us answer the really important question: what’s next?

So here, gleaned from the IPCC’s briefing, are some of the things we’re still trying to work out:

What’s up with clouds?

We’ve touched on this one before, but it’s just as true as ever: we don’t really know what’s going on with clouds. We know that they’re important in determining the “climate sensitivity,” the measure of how much warming you’d expect for a given increase in greenhouse gases. And they’re also obviously relevant to figuring out how the weather will be affected. But, as the IPCC says, trying to make clouds in a computer model is tricky.

The southern hemisphere

The bulk of long-term scientific research has been focused on the northern hemisphere, and those gaps in the observation grid mean that we know less about how things work down under.

We’re not quite as sure how all the extra energy in the Earth’s atmosphere, trapped by greenhouse gases, is warming the air in the Southern Hemisphere. This isn’t to say it isn’t warming. The question is about how much warming we’re seeing at different altitudes.

We’re also not quite sure how the rain has, or will, change. We know that over the northern hemisphere rainfall has been going up, but we’re not so sure what’s going on over the ocean or in the southern hemisphere.

Changes in Antarctic ice

The vast Antarctic glaciers are a focus of a lot of research, but we’re not really sure how they work. Scientists are trying to figure that out, because all of that ice could mean a lot of sea level rise. National Geographic says that if Antarctica and all the other ice melted we’d get something like 216 feet of sea level rise. (This is never going to happen, but it’s not fun to think about.)

We also don’t know as much as we’d like about the gigantic floating sheets of ice that ring Antarctica. Scientists are having trouble understanding why they sometimes seem to be growing, and there’s a lot of uncertainty in our predictions of what will happen to them as the world continues to warm.

Arctic permafrost bomb

The Arctic reaches of Canada and Siberia and Scandinavia and other polar regions are full of permafrost—land that’s frozen year round. As the world gets warmer, it makes sense that this permafrost will start to thaw (and it has been). What people are really worried about is that, trapped within this frozen soil, there is whole lot of carbon in the form of decaying plant material known as peat.

Peat likes to catch on fire. Peat also releases carbon dioxide and methane as it breaks down. So, there’s a big worry that if we keep thawing out the frozen peat, that there will be a big surge in greenhouse gases. But that’s exactly what it is—a worry. We’re not really sure how much extra greenhouse gases will be released from all this frozen land. A lot of it depends on how much we can limit global warming.

The power of the sun

Some people like to claim that changes in the amount of energy coming from the Sun are what’s actually causing climate change, and that greenhouse gas emissions aren’t to blame. If it’s all the Sun’s fault, then we’re off the hook. Those people are wrong.

That being said, of course changes in the amount of energy coming from the Sun affect the climate. How this happens, though, is the question. Scientists think that there may be a connection between the 11-year solar cycle and medium-term changes in the climate, changes that happen from decade to decade. This matters because these decade-to-decade changes can stack on top of the long-term changes caused by anthropogenic climate change.

The fate of the AMOC

There’s a gigantic circulation system running all throughout the world’s oceans, linking them together, transporting nutrients and salt and heat between the Pacific and the Atlantic and the Indian and the others. The Atlantic Ocean branch of this system is called the Atlantic Meridional Overturning Circulation—meridional because it flows “along the meridian,” and overturning because, from north to south, it flows along the bottom of the ocean, and from south to north, it flows along the top. This circulation system is very important for keeping everything moving, and its behavior affects everything from the temperature in Europe to the strength of the monsoon in China.

Scientists are worried that if climate change melts enough of the ice in Greenland and the rest of the Arctic that this circulation pattern could slow down, or even stop entirely. The IPCC says it’s “very unlikely” that the AMOC will stop in the next 100 years, but, after that, they’re not so sure.

What’s the takeaway here? We’re already locked in to a certain amount of climate change, thanks to the greenhouse gases we’ve already let into the air. We know that the world is going to change, but in some cases we’re not quite so sure what exactly is going to happen. We know a lot about climate change—we know that it’s happening and that it’s our fault—but that doesn’t mean scientists can take a break. There’s still a lot of work to be done to understand how the planet’s going to react to these changes we’ve wrought.