Human activity is warming the planet. How much it will warm by the end of this century depends on our choices now.

If we continue to emit a large amount of climate pollutants, global average temperatures could rise between 4.7 degrees F to 8.6 degrees F (2.6 degrees C to 4.8 degrees C) by the end of the century, compared to the average temperature around the end of the 20th century (1986-2005).

Humans, plants, and animals are already feeling the impacts, which will grow over time. Warmer air and water are causing sea level to rise and extreme weather events to be more frequent and intense. Rising carbon dioxide concentrations also are making the oceans more acidic.

Climate impacts are not evenly distributed across the globe nor within the United States. Each degree C of warming is expected to cause about a 1 percent drop in U.S. Gross Domestic Product (GDP), but some regions will likely experience much larger economic losses due to crop productivity losses, coastal damage, and deaths from heat exposure, among other factors. Parts of Florida and the Gulf Coast could see their economies shrink 25 percent if global emissions go unchecked and if local adaptation actions are not implemented. Warming is also likely to further exacerbate economic inequality among regions of the United States.

Although reducing emissions now can avoid the worst consequences, some impacts are unavoidable. Changes in the climate will continue until we drastically reduce our emissions. The good news is that scientists now believe that if the world were to have net zero emissions today, the Earth’s average temperature would remain fairly constant thanks to Earth’s carbon sinks, like the ocean and plants.

Sea-Level Rise

Warming increases sea levels directly (water expands when it warms) and indirectly (by melting glaciers and polar ice sheets). Global sea level has risen about 8 inches in the last 100 years, making coastal storms more damaging and accelerating erosion. Sea level is projected to rise an additional one to four feet by 2100, and more if glaciers in Greenland or Antarctica melt especially quickly.

Regions experience different amounts of sea level rise due to ocean mass distribution, which is affected by prevailing winds and ocean currents. Locally-observed sea level is also dependent on vertical land motion including rising land surfaces due to tectonic activity or the removal of mass from melting glaciers, as well as the loss of land in areas experiencing erosion. The U.S. Gulf Coast and East Coast have experienced some of the fastest rates of relative sea level rise in the world (0.5-2 mm/year) due to processes like sediment compaction, a natural process that can be accelerated by human activity like the extraction of groundwater and oil/gas. As the seas continue to rise, coastlines recede inland, further exacerbating flood risk in already exposed regions.

See the interactive NOAA Sea Level Trends Map and the U.S. Climate Resilience Toolkit Map.

Extreme weather

Warmer air temperatures increase the likelihood of severe weather events, just as using performance-enhancing drugs increases a baseball player’s chance of hitting a home run. Scientists see this effect in weather events today.

In many regions around the world, extreme precipitation events have produced more rain and become more common since the 1950s. In the United States, the Midwest and Northeast have exhibited the strongest increases in the amount of rain falling in heavy precipitation events, increasing the risks to life and property from flooding.

Extreme heat events have also become more common. As average temperatures have increased over the last century, the hottest days are hotter than they used to be. In the United States, more daily high temperature records are being broken, and fewer daily low temperature records are being broken.

More parts of the country see days with temperatures above 95°F. Hotter temperatures increase the risks of wildfire, drought and disease.

Ocean changes

Ocean acidification: Carbon dioxide (CO2) from the atmosphere is absorbed by the oceans, creating acid in the process. Acid concentration is measured as pH (on a scale of 0 – 14), where the more acidic something is, the lower its pH value. As atmospheric concentrations of CO2 have increased, the ocean has absorbed more carbon dioxide and created more acid.  The blue line in the figure below shows how pH has gone down (the ocean has become more acidic) at the same time that atmospheric CO2 concentrations have gone up (the red line). This trend is concerning because organisms like oysters, clams, and other shell-building species rely on carbonate to build their shells, which becomes less available in more acidic ocean water. With higher levels of acidity, ocean water can even dissolve shells. In the Southern Ocean, the shells of pteropods, a species critical to the ocean food web, have already been weakened by increased acidity.

Coral bleaching:  Corals are sensitive to temperature, light, and nutrient conditions. When these conditions fall outside of corals’ necessary range, the corals eject the microscopic organisms, called zooxanthellae, that symbiotically provide food via photosynthesis and give them their color. This process can be deadly to the coral and destroys a key home and feeding ground for other marine life. Recently, rates of coral bleaching have increased due to rising ocean temperatures, with one study finding that the interval between coral bleaching events has halved between 1980 and 2016.

Human Health

Climate change is not just an environmental problem; it’s a human health problem. Each year, extreme heat kills more Americans than hurricanes, tornadoes, or floods. Warmer temperatures affect air quality by accelerating the formation of ozone that can aggravate asthma and other respiratory illnesses. A changing climate can also expand the range of insects and ticks, which can be disease vectors for Lyme Disease, West Nile Virus, and Dengue Fever. Climate change also will increase the frequency or severity of certain extreme weather events, which can be deadly. As with many health impacts, young children, the elderly, those who are already sick, and those with limited access to healthcare are most at risk.

National Security

The U.S. military is concerned about direct climate impacts, such as how sea-level rise affects U.S. military bases along the coast. But military leaders are also concerned about the ways in which climate impacts can contribute to destabilizing states, posing new security risks and challenges. For example, more frequent and intense droughts and floods can disrupt agricultural productivity and food supplies, forcing population migrations that contribute to conflicts. As a result, U.S. military forces could face new security and humanitarian needs. Climate change has been called a “threat multiplier” by military leaders.