Since We Don’t Know Whether and How Much People Might Cut Greenhouse-Gas Emissions, It’s Hard to Know Exactly How High the Temperature Will Go by 2100.
Yogi Berra is credited with having said that “predictions are hard to make, especially about the future.” It’s true, but climate scientists do their best anyway. Computer models can do a pretty good job of simulating real-world events, but they can’t ever replicate them exactly (you’ve probably noticed this firsthand if you’ve ever seen a computer-animated human in motion). Even well-understood phenomena, like the physics of flight, can’t be simulated perfectly, but aeronautical engineers know they can rely on flight simulations because they know how closely their models represent the real world.
Similarly, models of the planet’s climate can’t simulate today’s climate precisely, but they do a good job of approximating it (and they’re getting better, as the models are continually improved and tested). The places where models and reality disagree help point out the areas of uncertainty.
These uncertainties come from the fact that the local climate is influenced by many different things, including ocean currents, ice cover, vegetation and cloud cover, to name just a few. As temperatures rise, each of these can change, leading the other parts of the system to react, leading to still more changes. Right now, scientists can’t reproduce all of these changes in their models, so they have to make approximations. For instance, cloud cover is likely to change as the temperature goes up. The changes could lead to extra warming, or to some cooling, or some of both. Scientists aren’t certain at this point about what the total effect will be, because cloud processes take place on too local a scale for models to handle. The balance of observational evidence so far suggests that clouds are likely to cause some extra warming. But it’s unclear how large this effect will be.
Since the models can’t yet give a dead-on representation, scientists know they can’t represent the future climate flawlessly either. That’s why projections for warming and sea-level rise and other changes are given as ranges, not specific numbers. But even if the models were perfect, they still wouldn’t be able to tell us how much the climate will change over the next century to the number. That’s because the amount of climate change will depend on human actions. For the last several decades, human greenhouse-gas emissions have been the biggest human-caused force changing global climate.
The amount of climate change we can expect, therefore, depends on just how much CO2 (and other greenhouse gases) humans will emit in the future. Since no one can predict the choices individuals, businesses and governments will make, climate models cover their bases by running several different simulations to project rates of warming over the next century. Each simulation is based on a different scenario involving the world’s population, economy, technology and energy use. Those possible futures all translate into different amounts of emissions, so each scenario results in a different amount of climate change later in this century. Until about 2050, it doesn’t matter much what we assume about future CO2 emissions, because much of the warming until then will be a delayed result of past emissions and because the different emissions scenarios are pretty similar early in the 21st century.
The results of these different scenarios probably won’t come as a huge surprise: The models show that if we end up cutting emissions by a lot, our climate will change significantly less than if we stick to business as usual. As shown in the latest IPCC report, from 2007, the scenario based on the lowest emissions estimate led to about 3.2 degrees F of overall warming from 2000 to 2100. The scenario based on the highest estimate translated to 7.2 degrees F of warming.
Note, however, that different climate models come up with different numbers, even with the same emissions scenario. So the 3.2-degree F and the 7.2-degree F represent an average of what the models say. In fact, some of the models say it will be less than 3.2 degrees F on the low end and more than 7.2 degrees F on the high end.

It’s important to understand that climate models are constantly being improved. That’s partly because computers keep getting more powerful and software keeps getting more sophisticated. It’s also partly because our observations of the actual climate are improving, so modelers can test their models better. The 2007 IPCC report was based on the best available models and information at that time, but there have been plenty of improvements since then.
The point is that projections are always a snapshot of a science that keeps evolving. Today’s projections are more reliable than earlier ones, but as models continue to improve, we’ll get new projections that will make these obsolete.
An Imperfect but Still Pretty Good Prediction: Sea Level Will Rise Two to Six Feet by 2100. But That Could Change.
There’s no doubt that sea level should rise if Earth’s average temperature goes up. For one thing, water expands when it warms up; warmer water simply takes up more space than cooler water. For another, mountain glaciers and ice sheets (like the ones covering Greenland and Antarctica) will tend to melt more in summer than they do now because summers are likely to be warmer in the future. (Ice floating on the sea, like the ice that covers the Arctic Ocean, will also melt. But that doesn’t add to sea level, any more than melting ice cubes make a drink overflow.)
How high sea level will be in 2100, though, depends on how much warming there will be. Since scientists don’t know exactly how much the planet will heat up, they can’t say exactly how much sea level will rise. The best they can do is give a range of likely numbers, and as of now their best projections range from about two feet at the low end to six feet at the high end, on average. This doesn’t mean it couldn’t be less or that it couldn’t be more. It just means anything less than two or more than six feet is less likely.
There’s one more factor that goes into sea-level projections besides expanding water and melting ice. Ice doesn’t just melt; it also flows very slowly (at a glacial pace, you might say) from the ice sheets covering Greenland and Antarctica down to the sea, where it eventually breaks off into icebergs. Glaciologists (scientists who study glaciers) have known for years that as the world warms, those glaciers might move faster. Water trickling down from the surface might make the rocks far below more slippery. Or warmer ocean water might melt some of the ice down where the glacier meets the sea, making it easier for the rest of the glacier to flow.
In 2007, when the most recent IPCC report came out, the authors had no climate model simulations available for them to assess that included the latest understanding of glacier processes. The report projected sea-level rise by 2100 at between six and a half inches and two feet but noted that this projection excluded future rapid changes in ice flow.
Very soon after the IPCC report was issued, satellite observations detected some of those changes starting to happen. At the same time, scientists who create virtual-reality climate simulations figured out ways to get a better idea of how sea level and temperature are related. Taken together, these new pieces of information helped climate scientists to come up with numbers they feel more confident about. A more recent report produced in the United States, called America’s Climate Choices, includes the more up-to-date two-to-six-foot range.
But those numbers could change in the future as glaciologists refine their understanding about how ice sheets respond to climate.
The Effects of Greenhouse Gases Won’t Magically Stop in 2100.
When scientists show graphs of increasing temperatures and rising sea levels, or project changes in extreme weather events, or talk about disruptions to ecosystems, they usually refer to what’s going to happen by 2100 — the end of the current century.
That might give the impression that 2100 is some sort of magic year — that the climate will be done changing, and we’ll be able to start adjusting to whatever the world looks like at that point.
That’s completely wrong, of course. It’s just a convenient date. People can wrap their minds around it, and, perhaps more important, it gives climate models a manageable cutoff point. If the models were called on to project conditions 200 or 300 or 500 years into the future, they’d take a lot longer to produce any results, and those results would be increasingly uncertain.
But if emissions continue to climb, and CO2 and other gases continue to build up in the atmosphere, global temperature will continue to rise along with them. If the buildup is still going on after 2100, the amount of heat trapped by greenhouse gases will continue growing as well.
In order to make 2100 a truly magic year — that is, a year when greenhouse gases are at their greatest level in the atmosphere — emissions would not only have to stop growing but would have to diminish to almost nothing. Climate scientists sometimes compare our atmosphere to a bathtub with a very tiny drain. For thousands of years, the trickle of CO2 entering the atmosphere from natural sources was balanced by the trickle that naturally drained out. Since the Industrial Revolution, when we started burning fossil fuels, we’ve opened the faucet wider and wider, so the water level (or, in the real world, the CO2 level) has risen.
If we simply stopped opening the faucet any wider, the water would still continue to rise. If we turned it down, the water would still continue to rise, just more slowly. And if we turned it back down to the original trickle — which would mean stopping all human emissions — the water level would stop rising.
But it wouldn’t fall, because that tiny drain is still tiny. Most of the extra CO2 we’ve added would stay in the atmosphere for a very long time. As a result, the planet would very likely remain in a state of higher temperature, altered weather patterns, higher sea level and the rest for at least 1,000 years, and the world would probably keep changing. The ice on Greenland, for example, is already shrinking measurably. If the temperature goes higher, as it almost certainly will, and if it stays higher for hundreds of years, the ice will keep on shrinking long after 2100.
All of this imagines that we turn off the human emissions faucet completely. Since it’s hard to conceive of how that could happen, the idea that 2100 will mark some sort of milestone — which scientists have never meant it to be in the first place — is clearly not correct.
It’s also important to point out that temperatures keep rising even after atmospheric CO2 stops increasing. That’s because it takes a long time for the ocean to adjust to the atmospheric CO2 concentration.
Best Guess About Atlantic Hurricanes in the Future: Fewer, but More Powerful.
Hurricanes draw their power from the heat in tropical ocean water. Sea surface temperatures in the part of the Atlantic where hurricanes form and intensify have been rising for the past century or so, at least partly because there’s more and more heat-trapping CO2 in the atmosphere.
Put those together, and you’d probably expect the number of hurricanes to be on the rise as well. You’d probably also expect the average hurricane today to be more powerful than it was decades ago.
But that’s not necessarily so. Hurricane formation is a very complex process. You need warm oceans, but you also need favorable winds. If the wind is blowing in different directions or at very different speeds in different layers of the atmosphere, hurricanes can have a hard time getting started even if the ocean is very warm. Differences in ocean surface temperatures between different basins — the Atlantic versus the Pacific, for example — also have an effect on hurricane formation.
So while the Atlantic is getting warmer over time, and while the number of Atlantic hurricanes has grown in the past 20 years, it’s not clear that rising CO2 levels have caused the increase. Scientists think that changes in other factors could be playing an important role. If you try to go back even further in time, using historical records, things get even harder to unravel. That’s because some hurricanes are born and die without ever touching land. The only way to be sure you’re counting them all is with satellites, and we’ve only had those available since the late 1960s.
It’s also not clear what might happen over the coming century as the oceans keep warming. The best computer simulations that now exist still can’t capture all of the complexity of hurricane formation, so it’s unwise to count on them too much. But thus far, they tend to point to a future where we would actually see fewer hurricanes in the Atlantic.
The simulations also say, however, that even with fewer hurricanes overall, the most destructive storms — known as Category 4 and 5 hurricanes, with wind speeds over 131 miles per hour — could get more common and more intense. Again, this is still a provisional result, so we’ll have to wait for better simulations and more observations before the question can be considered settled.
When a hurricane sweeps across the Atlantic and slams into the Eastern Seaboard or the Gulf Coast, as Irene did in August 2011, the amount of damage it does and the number of people it harms depend on where it strikes, not just on how powerful it is. If it comes ashore in a place where not many people live — as Hurricane Alex did when it struck about 100 miles south of Brownsville, Texas, in June 2010 — it doesn’t do a lot of harm. When it makes an almost direct hit on a major city (Katrina, 2005), it can be expensive and deadly. But even if the Atlantic were to see a decrease in strong hurricanes, damage and death would likely get worse anyway because more and more people are living in harm’s way.
In 1960, about 14 million Americans lived along coastlines where hurricanes are known to strike. By 2009, that number had jumped to more than 32 million. More people lived in Harris County, Texas (which includes Houston), in 2005 than lived along the entire Florida coast 50 years earlier.
And so on. You can find dozens of statistics like that, not just for the United States, but for the rest of the hurricane-prone world as well, including much of the Pacific coast of Asia (where they’re called typhoons) and the northern Indian Ocean. In all of these areas, population has grown dramatically over the past half century, but the growth has been greatest in major cities along the coasts — where hurricane or typhoon danger is the greatest.
If more people and property are in hurricane-prone regions, more damage will likely occur, whether or not hurricanes get more powerful. If hurricanes become more powerful, as current research suggests, so much the worse. A Category 5 hurricane has 80 percent more destructive power than a Category 4, so even if the number of hurricanes goes down, the damage can still be greater overall if the number of very intense hurricanes increases.

Whatever Happens with Hurricanes, Higher Sea Level Will Make the Storm Surges They Cause More Destructive.
Most people probably think the greatest danger from a hurricane lies in the storm’s punishing winds and torrential rains. But those who live on hurricane-prone shores, including the U.S. coastline from Texas to North Carolina, know there’s something else to worry about. It’s the storm surge, a huge pulse of seawater pushed up onshore by the oncoming hurricane, like a slow-moving tsunami. Just like a tsunami, a storm surge can pose a terrible threat to people and buildings. Much of the devastation caused by Hurricane Katrina in 2005, for example, came when the surge of Gulf water overwhelmed the protective levees to flood the city.
As Earth warms up, it’s possible that hurricanes could get stronger, but even if they don’t, the damage they do may get worse anyway. The reason is sea-level rise caused by melting ice and the expansion of seawater as it warms. On average, the world’s oceans are expected to rise by up to several feet by the end of the 21st century. If that happens, storm surges will have a built-in head start on any destruction they can wreak. Just imagine if sea level in the Gulf of Mexico had been several feet higher to start with when Katrina came along and pushed a wall of water ahead of it.
Of course this scenario assumes that all other factors besides sea-level rise remain the same. In reality many factors might not. For example, there’s the possibility that higher average sea levels will be coupled with fewer but more intense storms. If development along coastal areas was more strictly regulated, we could actually see less devastation to human populations in urban areas by hurricanes. Of course, in areas without strict building codes or other adaptation strategies, the dangers associated with intense weather patterns are all the greater.
In fact the insurance industry has already been thinking about the problem. In a study that came out in early 2010, it was estimated that sea-level rise will make storm surges cause 20 percent more property damage by the year 2030. But that’s if hurricanes don’t get any stronger and if no more buildings, roadways and other expensive construction happen along the coast. If hurricanes do get more powerful, or if more hotels and condos do go up near the beach, and no man-made or natural barriers (like barrier beaches) are in place, the damage will be even greater.
Adapted from the book "Global Weirdness: Severe Storms, Deadly Heat Waves, Relentless Drought, Rising Seas, and the Weather of the Future" by Climate Central. Copyright © 2012 by Climate Central. Reprinted with the permission of Climate Central.
Shares