Here are the negative repercussions scientists expect to see in a warmer world.
Direct effects of higher temperatures
Increased health risks. One of the most immediate effects of higher temperatures is an increased threat of health risks such as heat stroke. As noted above, this is probably the most easily understood risk.
Drought. There will be more droughts. For one thing, higher temperatures will lead to faster evaporation of surface water. For another, they will mean less snowfall, as precipitation will be more likely to fall as rain. In some regions, like much of the Southwest, flows of water through the spring and summer are a function of snow melting in the mountains. Reduced snowpack means less water later in the year.
Wildfires. Higher temperatures and drier conditions in some places will also help wildfires spread and lengthen the wildfire season overall.
“The things [scientists] are most confident about being able to predict for the future that come from increases in temperature,” Halliday said, “are increases in drought and increased probability of wildfires.”
Shifts in growing areas for crops. Higher temperatures are expected to shift growing areas for some crops. One researcher in Chicago is exploring how the Midwest, and not the South, may someday soon be the best growing environment for crops such as cotton. The growing area for corn is already shifting north, as well.
Broader spread of some diseases. One of the benefits of living in a colder-weather climate is that winter kills insects such as mosquitoes. As temperatures stay warmer, mosquitoes will be more likely to survive. Diseases that are commonly associated with the tropics will become more common farther north.
The cold doesn’t kill only mosquitoes. Increased temperatures will also allow other microbes to survive in regions where they would previously have been killed off.
Increased moisture in the atmosphere
Big precipitation events will be even rainier and snowier. One of the odd things about climate change is that the effects are expected to be radically different in different places. Places that are dry are expected to get drier; places that are wet, wetter.
Warmer air holds more moisture. So when there are large precipitation events, such as major storms, there’s more water in the air that can fall to the ground. This is one of the reasons that people point to the role of climate change in the development of Hurricane Harvey last year in Houston.
Halliday noted, though, that predicting where rainfall will increase has “been one of the hardest parts of climate change.” In part that's because the warming climate is expected to change how weather circulates. Harvey was as devastating as it was because the storm mostly stopped moving.
"What we're seeing, we think, is just a change in the ability of the atmosphere to move the clouds further inland,” Halliday said. The science on these changes is still in its early stages.
Warmer temperatures. Atmospheric moisture itself helps trap heat, making the problem of climate change itself worse.
Increased flooding. The increase in precipitation in some places will lead to more inland flooding. New York and parts of New England were devastated by flooding after Hurricane Irene made landfall in 2011.
Changes in the oceans
Rising sea levels. The more obvious flooding risk comes from rising sea levels. This increase is already well-documented. Parts of southern Florida now flood frequently enough that it barely makes the national news.
Rising sea levels are a mostly a function of two things.
The first is that warmer temperatures melt glaciers. The problem here isn’t ice that’s in the ocean melting — the problem is melting ice that’s on land.
The second is warmer ocean temperatures themselves. Water expands as it warms, and that increases the volume of the water. An increase in ocean volume means higher sea levels.
Halliday noted that as with other predicted effects, such as heavier rainfall, the places where sea levels will rise the most vary. The National Oceanic and Atmospheric Administration has a map of how sea level changes differ around the world. In some places, especially farther north, sea levels have fallen. (In some instances, Halliday noted, that’s because glaciers have melted, reducing the weight pressing down on a land mass and allowing it to rise.)
The combination of greater storm precipitation and higher sea levels is the reason the effects of major storms will continue to be dramatic. Halliday noted that the joint phenomenon of higher seas and heavy rain contributed to the widespread damage seen in the New York City area after Hurricane Sandy in 2012: Storms remaining relatively motionless as they empty out onto cities makes the potential for disaster much worse.
About 40 percent of the world’s population lives within 60 miles of the ocean.
Warmer ocean water. Speaking of storms, warmer ocean water contributes to the severity of storms such as hurricanes. That warmth helps fuel tropical cyclones and is one reason we’ve seen recent storms become so big so fast.
Ice melt. This is also a problem in the Arctic. Warmer water combined with warmer temperatures helps melt sea ice more quickly. While this doesn’t contribute to sea level rise, it does shift animal habitats and affect people in the region. In 2015, The Post’s Chris Mooney profiled a town in Alaska that needs to be relocated since thinner sea ice has left it more vulnerable to large waves.
Melting Arctic ice, as Mooney noted, also exacerbates the warming problem. You’ll remember from middle-school science class that lighter colors reflect sunlight, while darker colors absorb it. Ocean water is much darker than ice, so it absorbs more energy from the sun.
Shifts in sea habitats. Another effect of warmer ocean water is that it affects sea life, much as warmer air temperatures affect crops. Lobsters, for example, are increasingly headed north.
More-acidic oceans. Carbon dioxide, the most common greenhouse gas, doesn’t end up only in the atmosphere. The oceans are also absorbing a lot of the carbon dioxide that’s produced when we burn fossil fuels, and that is making the oceans more acidic. That acidification has a negative effect on some sea life, including dissolving the shells of some ocean animals.
The combination of increased acidification and warmer water is what poses the greatest risk to the world's coral reefs.
Climate change will make itself worse
There are examples of this above, like the melting of ocean ice itself leading to increased energy absorption and warmer ocean water. But the most significant negative feedback is from a different sort of melting.
Thawing permafrost. Some areas in the Arctic that are normally frozen year-round have begun to thaw. That includes a lot of vegetation that, once thawed, begins to rot. That rotting vegetation will release a lot of methane — a greenhouse gas that’s much more effective at trapping heat.
The thawing of permafrost, Halliday said, could reach a tipping point where “you suddenly accelerate a change” that increases warming much more rapidly than previously expected. The result could be “quite catastrophic,” he said, including triggering a chain of other tipping points that themselves accelerating warming.
The report on climate change that the United Nations released Sunday addressed a fairly specific rate of warming, contrasting it with possible — or likely — additional warming. The difference between an increase in 1.5 degrees Celsius and 2 degrees Celsius is depicted well in this interactive from the New York Times. But the effects of a chain reaction of tipping points could be much worse still.
“The degree to which that’s certain? It’s not certain at all,” Halliday said of the permafrost scenario. “The degree to which you should worry about it? You should seriously worry about it a lot. Even if it’s just a 20 percent chance of this happening, you’d never get on a plane if you only had an 80 percent chance of getting to the other side.
“I think we need to be very worried about these things that we’re not very certain about at the moment,” he said. As we learn more, he added, we learn more about the “potential that actually it could be far worse than we’ve actually considered before. And that has important implications for everything: For what will happen to the economics of the world, what will happen to populations, what will happen to food security, etc.”
That, in essence, is the most precise summary of the effects of climate change. All of the factors above, yes, but more broadly that the effects will be bad, they will be substantial — and they may very well be worse than we expect.