A degree or so of global warming sounds small. You would barely notice it between one afternoon and the next. But a global average hides how the change reaches people: through the extremes. This lesson looks at how a warmer climate changes heat waves, rain, drought, storms, coastlines and ecosystems, and how scientists can now link specific events to climate change.

Shifting the odds

Picture the daily temperatures in a place as a bell curve, with most days near the middle and a few extreme days in the tails. Nudge the whole curve slightly warmer and something dramatic happens at the hot end: days that used to be rare become common, and new extremes appear that were almost impossible before. The IPCC estimates that a heat extreme that struck once every 50 years in a pre-industrial climate now happens about five times as often. At 1.5°C of warming that rises to about 9 times, and at 2°C to about 14 times.

Water: too much and too little

Because warmer air holds about 7 percent more moisture per degree, the heaviest downpours are becoming heavier in most regions where good records exist. At the same time, more heat speeds evaporation, drying soils and stressing crops in regions prone to drought. Mountain glaciers and snowpack act as water towers for well over a billion people; as they shrink, the summer meltwater that rivers depend on will eventually dwindle.

For tropical storms such as hurricanes, the evidence does not show more storms overall. It does show that a larger share reach the most intense categories, that they carry more rain, and that the same storm surge now rides on top of higher seas.

Attribution science

For years scientists said that no single weather event could be blamed on climate change. That has changed. Extreme event attribution compares how likely an event was in today's climate with how likely it would be in a simulated world without human emissions. A 2004 study found that human influence had at least doubled the risk of a heat wave like the one that struck Europe in 2003, which killed tens of thousands of people. In June 2021 a heat wave in the Pacific Northwest pushed the village of Lytton, British Columbia, to 49.6°C, a Canadian record. The next day the village burned in a wildfire. The World Weather Attribution group found the heat wave would have been virtually impossible without human-caused warming.

Oceans and ice

Sea level will keep rising for centuries as the deep ocean warms and ice sheets respond. The IPCC's 2021 likely ranges for 2100 run from roughly 30 to 55 centimeters of rise under very low emissions to roughly 60 centimeters to 1 meter under very high emissions, compared with the start of this century. Rises toward 2 meters cannot be ruled out if ice sheets behave in ways that are still poorly understood. Even modest rises make flooding from storms and high tides far more frequent on low-lying coasts.

Marine heat waves have grown more common. Coral reefs, which shelter about a quarter of all marine species while covering a tiny fraction of the seafloor, are especially vulnerable. When water stays too warm, corals expel the algae that feed them and turn white, a process called bleaching. Repeated mass bleaching has struck reefs around the world in recent years. The IPCC projects that warm-water reefs would decline by 70 to 90 percent at 1.5°C of warming and by more than 99 percent at 2°C.

Unequal burdens

Impacts fall unevenly. People who are poorer, older, work outdoors or live in places with little infrastructure face greater danger from the same heat or flood. Many of the most exposed countries have contributed the least to emissions. These questions of fairness belong mainly to ethics, but the science helps show who is at risk and why.

Worked example: is half a degree worth worrying about?

Is the difference between 1.5°C and 2°C of warming large enough to matter? Compare the IPCC's 2018 special report findings.

• Extreme heat: a once-in-50-years heat extreme goes from about 9 times to about 14 times as frequent as before industry.
• Coral reefs: the loss goes from 70 to 90 percent to more than 99 percent.
• Arctic summers: an ice-free Arctic in September goes from roughly once a century to roughly once a decade.
• Sea level: about 10 more centimeters of rise by 2100, enough to expose around 10 million more people to related risks.

Each step of warming makes the next one worse. The useful question is not whether a line has been crossed but how much harm each tenth of a degree adds, and the answer is a lot.