One of the clearest predictions from climate change modeling is warmer temperatures. Almost everywhere in the world will be warmer by 2100, and we’ll see more extreme heat. The amount of warming we'll get depends mainly on the amount of fossil fuels we burn.

The black line on this graph shows the average of a set of temperature simulations for the 20th century. The colored lines show projected temperatures for the 21st century based on three emissions scenarios. Shaded areas around each colored line indicate the range of likely outcomes based on running the climate model many times. See the full interactive graph at climate.gov. Source: NOAA’s Climate.gov

Choosing Our Future

In predicting the climate of the future, the biggest unknown is what humans will do. How will we change our use of fossil fuels—and thus our CO₂ emissions? The answer involves individual and collective decisions at local, national, and international levels.

To understand the consequences of the paths we might take, scientists use climate models. They input different amounts of future CO₂ emissions to see how they affect the climate.

RCP stands for representative concentration pathway, meaning a pathway that assumes a certain concentration of carbon dioxide in the atmosphere. The number in each scenario name indicates how much heat would be added to the atmosphere. Source: Based on data from the 2013 IPCC report

How Would That Feel in Sacramento?

Here’s what high emissions, the RCP 8.5 scenario, would mean for Sacramento, California’s state capitol. The average high temperature for the whole year, including winter, would be ten degrees warmer, rising from about 74ºF (in the 1960s through 1980s) to about 83ºF by 2100.

Not only that, there would be about 10 times as many extreme heat days (heat in the 89th percentile)—about 40 days per year in 2100, compared with only four days in the 1980s. There would also be about six heat waves per year, instead of one, and these heat waves would last longer. (A heat wave here means four days in a row of 104ºF or higher.)

Observed temperatures are shown until 2005, followed by data from four climate models for 2006 through 2099. The gray band shows the possible variability in the modeled future. Source: Based on graph from Cal Adapt, the California Energy Commission

Climate change models predict that some parts of the world will get more rain and snow than the historical average, while others will get less. Different climate models give slightly different projections, yet there’s one thing nearly all the models predict: intense rainstorms will get even more intense, wreaking havoc through flooding and erosion.

How much will rain and snow increase or decrease in the future? This animation shows climate model predictions of rain and snow for different parts of the world, from 2006 through 2100. Blue colors mean more rain or snow than average (based on 1971–2000), tan to orange mean less. Source: NASA's Goddard Space Flight Center Scientific Visualization Studio

Wetter in India, Drier in the Mediterranean

A few regions are predicted to get more rain or snow. In the United States, the Northeast will likely get wetter, as will Alaska. The Pacific Ocean near the equator will be much wetter, as will India and areas near the North and South Poles, such as Canada, Russia, and northern China.

The Mediterranean Sea region will get much drier—including Spain, Portugal, Italy, Greece, Turkey, and North Africa’s coast. Southern Africa—South Africa, Namibia, and Botswana—will be much drier, as will southwestern Australia.

The outlook is not so clear for the rest of the globe—different climate models give different results.

When It Pours, It Will Really Pour!

One thing all the climate models agree on: big rainstorms will be get even bigger, dumping lots more rain. In the United States, this is most likely in the Northeast and the upper Midwest. This could increase the chances of river flooding, especially along the coast where sea level is also rising. Weather records show that extreme rains are already beginning to increase.

Predictions of how much more often extreme rainfall or snowfall events will happen at the end of this century (2081 to 2100) under the high carbon dioxide emissions scenario (RCP 8.5). The darkest color in the continental United States means these events would happen five times as often as they did at the end of the 20th century. Source: Based on NOAA National Climate Data Center image

Flooding of the Souris River in Minot, North Dakota, inundated more than 4,000 homes in June 2011. Source: U.S. Air Force

California’s Future: Snow Turns to Rain

Climate models don’t always give a clear picture of the future, and precipitation in California is a good example. Some models say the state will get more, some say less, and others say it will stay the same.

What’s more certain: snowpacks in the Sierra Nevada mountains will all but disappear, as warming temperatures cause precipitation to more often fall as rain instead of snow.

That’s bad news for California’s water supply. The state relies on mountain snowbanks that melt throughout the summer, yielding steady flows of water to fill the reservoirs. Without snow, managing water will be much trickier.

California’s snowpack will shrink by two-thirds by 2050, according to the climate models, as more precipitation falls as rain instead of snow. This animation shows 10-year averages of the April snowpack from the 1950s through the 2090s, according to a high CO₂ emissions scenario. Source: Cal Adapt, California Energy Commission

Climate change is already causing sea levels to rise. And climate models predict this will continue, and may even accelerate in the future if carbon emissions increase. Sea level rise will threaten low-lying islands, towns, and cities around the world with catastrophic flooding. Those in the most vulnerable spots are already planning to relocate. In large metropolitan areas, like the Bay Area, governments are planning ways to cope with the threats to infrastructure, such as highways, railways, and sewage treatment plants.

In this map of future sea level rise in the Bay Area, blue areas are projected to be flooded—that is, below sea level—by the year 2100, assuming a high carbon dioxide emissions scenario, and assuming we don’t build or raise any barriers. Green areas would also be below sea level, but are currently protected by ridges, levees, or other barriers. Click the image to zoom in or explore how other places will be affected. Source: Climate Central

How Does Climate Change Cause Sea Level Rise?

It’s a combination of two factors:

1. Rising temperatures melt glaciers and ice caps, sending more and more water into the ocean.

2. Warmer ocean water takes up more space. Just like the liquid in a thermometer, water expands as it gets warmer. Each drop of ocean water expands just a bit, but when you combine all the drops in the ocean, it’s enough to raise sea levels.

Austria’s Pasterze Glacier is retreating about 33 feet (10 meters) a year, and has lost half its volume since 1851. Source: H. Raab

How High Will It Get?

If we continue on our current path—getting most of our energy from burning fossil fuels—sea levels are projected to rise an additional 0.7 meters (2.5 feet) on average by 2100, though it could rise by as much as 1 meter (3 feet). That’s according to climate model predictions from the IPCC in 2014—some later studies suggest that we should double those numbers.

The longer we delay the transition toward renewable energy sources, the more likely this outcome will be. If we switch to renewables sooner, sea levels could rise less.

Sea level has been rising since the mid-1800s, when people started burning fossil fuels, such as coal. It began rising faster around the 1990s. The pink line shows future rise projected for the high emissions scenario—our current path; the pink area is the range of uncertainty. The blue line and light blue area represent a very low emissions scenario. Sea level rise at 2100 is different from what's listed in the text above because this graph is compared to the early 19th century instead of the late 20th century. Source: Based on 2013 IPCC graph

Frequent Flooding to Come

A few feet of sea level rise might sound manageable, especially if you don’t live right along the shoreline. But the levees, sea walls, and other barriers that protect coastal cities are only designed for ordinary high tides and storm waves. With sea level rise, catastrophic flooding that used to happen once in a decade or once in a century could be a regular occurrence.

Who’s at Risk?

More than 100 million people worldwide may be exposed to inundation by 2100, according to climate research. The most vulnerable communities often don’t have the financial resources to cope with this flooding.

Some South Pacific island nations, such as Kiribati, are already giving up the battle against sea level rise. Kiribati President Anote Tong said in 2008 that the country had reached "the point of no return." He added, “the emissions in the atmosphere will carry on contributing to climate change, to produce a sea level change so in time our small, low-lying islands will be submerged.”

On Tarawa Island in Kiribati, a wall temporarily protects a small house from the sea. In 2014, the Kiribati government purchased land in Fiji, and is actively urging citizens to abandon their low-lying island nation.

Urban Infrastructure

It’s not only coastal homes that are threatened. Highways, railways, airports, and sewage treatment plants are often built directly along shorelines, especially in the Bay Area. And they’re protected by relatively low, poorly maintained levees.

On this map of Hayward, at the east end of the San Mateo Bridge, blue shows land that would be flooded by four feet of water level rise. Threatened areas include Highway 92 leading to the Bridge, as well as a sewage treatment plant at the shore just north of the area shown. Source: Bay Conservation and Development Comission (BCDC)

More Sites About Sea Level Rise

• When Rising Seas Hit Home: An analysis by the Union of Concerned Scientists
• Surging Seas: Sea level rise analysis and dynamic mapping tools
• Days with High-Tide Flooding: From the U.S. Climate Resilience Toolkit, predictions from climate models
• King Tides and Climate Change (video): What today's highest tides show about sea level rise

If carbon emissions continue to grow quickly, the weather is predicted to change dramatically, with most weather events becoming more extreme. Stronger hurricanes, longer droughts, more and wetter thunderstorms, and more catastrophic flooding are all among the results from climate models.

Soil moisture levels from 1950 through 2095, based on climate model results for a high carbon dioxide emissions scenario (RCP 8.5). Brown means less moisture than average, blue means more. The Southwest is projected to get much longer and more severe droughts than at any time in recent memory. Source: NASA’s Scientific Visualization Studio

Heat Makes Drought Worse

We think of drought as a lack of rainfall. But the warmer temperatures projected by climate models could cause drought simply by drying out the soil through evaporation.

The map above shows how much dryer the soil is expected to be. Mexico and the “four corners” states—Utah, Colorado, Arizona, and New Mexico—are projected to get both less water and higher temperatures—the perfect recipe for epic drought.

Different Catastrophes for Different Areas

Just as the weather’s a bit different wherever you go, the projections of extreme weather vary all around the world. For example, the Southwest can expect more intense and frequent droughts, Florida can expect more catastrophic flooding, and over the Atlantic Ocean, hurricanes are expected to be more intense, bringing much more rain.

Climate models predict that by the year 2080, the risk of flooding will increase in the pink areas, and decrease in the green areas. In the darkest pink areas, such as Southeast Asia, a catastrophic flood that used to happen once in 100 years will happen every 15 years. Source: Based on IPCC image

Warmer Waters Feed the Hurricane Machine

The hurricanes that batter the eastern United States and Caribbean begin far away, in the tropics of the Atlantic Ocean. As climate change warms the oceans, more water will evaporate into the atmosphere, adding power to newborn tropical storms.

More than 20,000 homes were flooded in Port Arthur, Texas, by Hurricane Harvey. In total, the storm caused more than $125 billion in damage, mostly in the Houston area, tying it with Katrina for the costliest tropical storm in history. Source: U.S. Army National Guard

Heat-Related Deaths

As extreme heat events increase, it’s very likely that we’ll see more heat related illness and deaths from conditions like heat exhaustion, and aggravated heart or lung illnesses, especially among the elderly and the very young

More Links About Extreme Weather:

• Extreme Weather Page from the 2014 National Climate Assessment
• Regional Projections for Climate Change from the 2014 National Climate Assessment