Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 10.8, Cambridge University Press.

Global average temperatures are expected to increase by about 2-13°F (1-7°C) by the end of the century. That may not sound like a lot, so what's the big deal? The problem is that small changes in global average temperature can lead to really large changes in the environment.

Only a few degrees have made the difference between ice ages, temperate periods, and a hothouse Earth with green poles. During the last ice age, global average temperatures were only 7-13°F (4-7°C) colder, but ice ploughed as far south as Illinois. About 125,000 years ago, average temperatures were only a degree or so warmer than today, but sea levels eventually rose as much as 20 feet (6 meters).

Small changes in the global average temperature also mean heat waves get hotter and droughts get drier. This happens thanks to small changes in the way air and water circulate around the globe. The resulting modifications to the environment can have large consequences for ecosystems. For instance, during the wetter and cooler climate of the last ice age, the Great Basin of California was filled by a great lake ringed with pine forests. When the climate became warmer and drier at the ice age's end, the lake dropped 500 feet and lost 90% of its surface area. The pine forests became sagebrush desert. Such changes in precipitation could be just as important, or even more important, than temperature increases for many parts of Earth.

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 10.4, Cambridge University Press.

These kinds of effects are not temporary, either. The climate is like a large ship: once it starts turning, it is hard to stop. Whatever the new direction is, it will affect your children, your grandchildren, and many generations after.

What will a changing climate mean for us in the future? Scientists make the best assumptions they can using models to project what might happen. But we don't know for certain what will happen or when because we don't know what choices people will make about changing energy sources, limiting population growth, preserving forests, etc. Some impacts are more certain than others, some will occur sooner and others later, and some will affect developing nations more than wealthier ones.

Climate and weather events now take place in a world that is different from the one our grandparents knew. The average temperature of the atmosphere is warmer. Oceans are also heating up, and polar ice is melting, as are many glaciers. Heavy bouts of rain and snow are becoming more frequent, with longer dry spells in between. Ecosystems are also changing. While day-to-day weather generally remains familiar, it plays out against a backdrop of change, and the mix of weather is slowly evolving as some events become more likely and others less common.

Extreme weather is a bit relative. For example, a hot day in Wyoming would be considered a mild one in Arizona. And we don't generally notice changes in climate because most weather falls within the range of what is expected. It is mainly the extreme events that get our attention—events that are outside our normal experience and that often inflict human suffering. So how might global warming affect climate extremes and extreme weather? There are several possible ways. Let's look at temperature as one example.

The normal distribution of temperature can be thought of as a bell-shaped curve like this, with the majority of the observations in the middle, but with rare events of extremely cold or extremely warm temperatures at the ends.

Global warming could shift the distribution to the right—decreasing the frequency of very cold weather events, but greatly increasing the occurrences of hot weather.

Another possibility is that climate change increases the variance in the distribution of temperature—in other words, it increases the range of possibilities at both ends of the distribution.

Or climate change could result in a combination of the two types of changes. We don't know exactly how the distribution of temperature will change, but most climate scientists strongly agree that, on the average, the global climate will continue to warm. However, the effects of this warming in specific locations are less known.

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 10.19 (b), Cambridge University Press.

Of course, there will always be natural variability, with some places and some years warmer or cooler than average. In general, however, summers will get hotter, not only because of higher temperatures but also because humidities will increase, raising the heat index, which is a measure of the effects of heat plus humidity on humans. A warming trend on top of natural variability means that temperatures will cross today's heat thresholds more often.

Number of days with temperatures hotter than normal during the 2003 European Heat Wave.

For example, the summer of 2003 was the warmest in at least the past 500 years in Europe and resulted in over 70,000 deaths. Researchers wanted to know if this heat wave was simply an extremely unusual natural event or whether climate change created an environment that altered the pattern of natural variability. Using a climate model, they analyzed the probabilities of exceeding the 2003 seasonal mean temperature under 2003 conditions, including the build-up of greenhouse gases from human activities. They then ran the model again without those greenhouse gases. Their conclusion is that global warming probably at least doubled the chances of the heat wave. Models predict that in many places, an extreme heat event that we see once every twenty years today will happen once every three years by the middle of this century, and even more frequently by century's end.

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental. Panel on Climate Change. Figure 10.19 (a), Cambridge University Press.

On the plus side, winters will be warmer in many places, reducing heating bills. And the number of days with frosts is likely to decrease. On the downside, it means more snow may fall as freezing rain, making ice more of a problem for motorists, pilots, and seniors.

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental. Panel on Climate Change. Figure 10.18 (b), Cambridge University Press.

Because a warmer planet will boost evaporation and change atmospheric moisture circulation, many dry areas will become drier and wet areas wetter. In North America, on average, precipitation is likely to be less frequent but more intense when it does fall.

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental. Panel on Climate Change. Figure 10.18 (d), Cambridge University Press.

This may mean that the American Southwest becomes drier. Indeed, we have already begun to see these trends in the past few decades. And along with increased droughts comes an increased potential for wildfires.

The oceans will continue their rise in the coming century. In their 2007 report, the IPCC estimated that the global average sea level will rise by about 9 to 20 in. (22-50 cm) by 2100 (relative to 1980-1999) under a range of greenhouse gas emissions scenarios. It is important to note that these estimates assumed that melting from Greenland and Antarctica would continue at the same rates as observed from 1993-2003. However, new research suggests that the IPCC predictions may be too low, and that sea level rise could be closer to 3 ft (1 m).

If seas rise two feet (0.6 meters), the United States could lose 10,000 square miles (26,000 square kilometers) of land surface, mostly on the Gulf and Atlantic coasts. If they rise three, they will inundate Miami and most of coastal Florida.

This map highlights regions that are most vulnerable to coastal flooding based on a scenario for the 2080s that assumes a global sea level rise estimate of 18 in. (45 cm). Nearly half of the world's population lives in low-lying coastal areas, and continued population growth along coasts increases vulnerability from sea-level rise, storm surge, and flooding.

Even a small amount of sea level rise can produce major changes for coasts. In low-lying areas, a foot and a half of vertical rise (0.5 m) can cause inundation far inland, as illustrated in this image, which shows how Charleston, South Carolina would be impacted by that magnitude of sea level rise.

Other effects of sea level rise can include saltwater intrusion into freshwater aquifers that provide drinking water and loss of coastal wetlands and their ecosystems. One estimate projects 17-43% of U.S. wetlands could be submerged, of which more than half would be in Louisiana. Though new wetlands will form farther inland, their total area will probably be reduced, which would put New Orleans and other coastal cities at even greater risk from hurricane storm surge.

Extreme weather events cover the spectrum from long-term droughts to short-lived tornadoes. When talking about the effect of climate change on different kinds of extremes, scientists have more confidence for some than others. But we do know that increasing greenhouse gas emissions are affecting temperature and precipitation patterns, which are the main drivers of weather events. So it makes sense that these changes can also alter the odds of an extreme event happening.

What Do You Think?

Studies suggest that for every 1.8°F (1°C) increase in tropical sea surface temperatures, core rainfall will increase by 6-18%, and surface wind speeds of the strongest hurricanes will increase 1-8%. Storm surge levels are also likely to increase. However, some studies suggest that vertical wind shear increases may reduce hurricane activity, at least in some areas.

We don't have enough data on tornadoes to determine whether their frequency or severity is changing. And global climate models can't currently simulate features as small as these storms.

While winters will be warmer overall, when storms do occur, they will likely produce more snowfall, stronger winds, and higher waves, although these effects may vary with location.

Of course, extreme weather events can have some positive effects. The warm winter of 2011-2012 in the U.S. resulted in lower heating bills, and many communities also saved by not needing snow removal services.

Often, it is an individual's or community's vulnerability to the adverse effects of extremes that largely determines whether a particular event is a disaster or not.