As the mercury creeps up over the summer, it gets harder to do anything other than curl up inside an air-conditioned room or dive into the ocean.

If you've noticed that recent summers have felt particularly hot, you're not wrong. The past four years have been the hottest four on record around the globe, with this year tracking to be the fourth hottest year ever. Heat in 2018 has already set all kinds of records, including the hottest temperature ever measured in Africa and the hottest overnight temperature ever recorded.

Unfortunately, that trend is expected to continue.

As humans continue to pump greenhouse gases like carbon dioxide into the atmosphere, more of the heat that our world absorbs from the sun gets trapped, raising the world's average temperature and triggering other changes.

By 2050, cities in the US and around the world are expected to see a skyrocketing number of days with temperatures topping 100 degrees, and temperatures are projected to climb even higher by 2100. New York City, which has an annual average of zero days above 100 degrees Fahrenheit now, is expected to see 11 days like that per year by 2050 and 30 such days by 2100. Houston, which currently sees two days that top 100, is expected to get 30 such days by 2050 and 76 by 2100.

That heat isn't just uncomfortable. The warming has serious effects on people's physical health, mental well-being, and cognitive ability.

Here's what science tells us about how extreme heat affects the body and brain.

Heat causes heat exhaustion, which can be dangerous.

Stepping outside on a July or August day can feel like a physical blow. The longer you spend in the heat, the more serious the effects on your body can be.

First, increased body temperature can start to cause heavy sweating, clammy skin, dehydration, tiredness, headache, dizziness, nausea, cramps, and a quick, weak pulse.

Someone in this state should move to cool place, sip water, and take a cool bath or put cool wet cloths on their body. If these symptoms last longer than an hour, worsen, or if a person is vomiting, then they need medical help, according to the CDC. 

 

Once a person gets hot enough, they can develop heat stroke.

Once body temperature rises to 103 Farenheit or higher, a person starts to suffer from heat stroke, which can be a fatal medical emergency.

Symptoms of this include many of the signs of heat exhaustion, though a person with heat stroke may have a fast, strong pulse; feel confusion; and may be losing consciousness. They also may stop sweating.

People suffering from heat stroke need to be cooled immediately. In that situation, don't give a person anything to drink. Move them to a cool place, put cool cloths on them or put them in a cool bath, and call 911.

Extreme heat makes us dumber.

If you've ever felt like the heat puts your brain into a fog — a sensation like that in a steam room, where it's hard to breathe, much less think clearly — you're not alone.

A number of studies show that as temperatures climb, we perform more slowly and more inaccurately on cognitive tests. This phenomenon affects everyone from students taking standardized tests to office workers trying to get through the day.

Heat causes air pollution and air quality to get worse, which makes it harder to breathe and leads to disease.

Ever noticed how you see more air quality alert days in the summer? Get ready for more.

On hot days, heat from the sun causes pollutants to react with atmospheric gases to form ozone. The hotter it is, the more ozone pollution is produced. Plus, still air on hot days causes smog to stick around.

One 2008 study found that for every degree Celsius the temperature rises, ozone pollution can be expected to kill an additional 22,000 people around the world via respiratory illness, asthma, and emphysema.

Non-ozone air pollution linked to warmer weather will also increase rates of lung cancer, allergies and asthma, and cardiovascular disease.

A 2017 study found that air pollution already kills 9 million people every year. So as temperature increases, that death toll will rise.

Abnormally high temperatures can cause suicide rates to spike.

A study published this month in the journal Nature Climate Change reported that a 1-degree-Celsius rise in average monthly temperature was associated with an increase in the monthly suicide rate. In the US, that increase was about .7%, and in Mexico it was 2%.

By 2050, the study authors concluded, this will likely lead to 14,000 additional suicides in the US, though they say there could be as many as 26,050 more.

 

Hotter weather causes mental well-being to deteriorate.

Many of us might associate the transition from winter to summer with a positive mood, but it seems the heat can wear us down over time.

The authors of that same study on the link between climate and suicide also analyzed more than 600 million tweets, and found that people were more likely to express depressive feelings as temperatures rose.

Warmer weather makes allergies and asthma even worse.

When spring arrives every year, allergy sufferers feel it in their noses, throats, sinuses, eyes, and more. Spring pollen season now begins earlier in the year, and the growing season for allergenic pollen like ragweed has gotten longer.

More carbon dioxide in the air also increases pollen levels.

All of this leads to more sneezing and sniffling for for allergy sufferers — and these allergy symptoms can also make dangerous asthma attacks more frequent.

Heat waves are the deadliest form of extreme weather, responsible for more deaths in the US every year than the combined effects of hurricanes, lightning, tornadoes, earthquakes, and floods.

A study published last year in the journal Nature Climate Change found that 30% of the world is already exposed to heat that's intense enough to kill people for 20 or more days each year. That level of intensity is defined using a heat index that takes into account temperature and humidity; above 104 degrees Farenheit (40 degrees C ), organs swell and cells start to break down.

In 2010, more than 10,000 people did in a Moscow heat wave. In 2003, some estimates say a European summer heat wave killed up to 70,000.

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