Scientists say the planet is hurtling toward a global-warming danger zone, and humans deserve much of the blame. But how exactly does burning coal or clearing forests lead to glaciers melting, seas rising and oceans overheating? Here's how experts describe the system.

It all starts with greenhouse gases

Greenhouse gas emissions In gigatons of CO 2 equivalents per year 12 10.7 China 8 U.S. 4 E.U. 3.2 0 2012 1990 Source: World Resources Institute

Greenhouse gases in the atmosphere insulate Earth like a cozy comforter. They allow the sun to warm the planet, then hold in some of the heat that would otherwise radiate back into space. Human activity since the Industrial Revolution has upset the balance of several atmospheric gases, such as methane, nitrous oxide and fluorinated gases. But the one that has really gone out of whack is carbon dioxide.

What does this gauge show?

China and the United States are the biggest emitters of greenhouse gases, followed by the European Union. Burning fossil fuels — coal, oil and natural gas — accounts for the largest portion of greenhouse-gas emissions; other culprits include agriculture, cement production, livestock breeding and deforestation.

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What’s the problem?

Human industry has belched so much extra greenhouse gas into the atmosphere that it is building up, like extra stuffing in that comforter. A thicker layer holds in more heat.

Greenhouse gas emissions from human activity, in gigatons of CO 2 equivalents 100 Current trajectory (not including large-scale burning of biomass) would probably result in a greater than 4°C rise in average global surface temperature. 80 Range of Range of uncertainty uncertainty Proposed pledges from the Paris summit still fall short. All human-caused 60 greenhouse-gas 54 emissions 52.7 42 40 Levels needed to avoid a 2°C temperature rise over preindustrial levels. 20 HISTORICAL PROJECTED 0 1990 2010 ’14 ’25 ’30 ’50 Source: U.N. Environment Program Greenhouse gas emissions from human activity, in gigatons of CO 2 equivalents Current trajectory (not including large-scale burning of biomass) would probably result in a greater than 4°C rise in average global surface temperature. 100 HISTORICAL PROJECTED 80 Range of uncertainty All human-caused greenhouse-gas emissions 60 54 52.7 Proposed pledged reductions from the Paris summit would bring levels to 54 by 2030, still far above what is needed to prevent a 2°C rise. 42 40 Levels needed to avoid a 2°C temperature rise over preindustrial levels. 20 0 1990 2010 2014 2025 2030 2050 Source: U.N. Environment Program

Carbon dioxide is the biggest problem

Atmospheric carbon dioxide In parts per million 398 400 375 350 325 316 300 Oct. 2015 March 1958 Source: NOAA Earth System Research Lab.

Carbon dioxide is produced naturally by many sources — every time we exhale, for instance. Oceans absorb it, and plants use it during photosynthesis. That makes for a nice give-and-take called the carbon cycle. But carbon dioxide is also a huge byproduct of industry, and it accounts for about 82 percent of U.S. greenhouse-gas emissions. Oceans and plants cannot absorb that much.

What does this gauge show?

Before the Industrial Revolution, the atmosphere contained about 280 parts per million of carbon dioxide. By 2015, the annual average was above 400 ppm, according to the World Meteorological Organization. Some climate experts say that level is already too high to avoid grim repercussions from global warming; others say we still have wiggle room.

[Holding warming under two degrees Celsius is the goal. But is it still attainable?]

What’s the problem?

Scientists say we have already burned two-thirds of the amount of carbon the atmosphere can handle before the planet warms beyond the “danger zone,” which many consider to be two degrees Celsius (3.6 degrees Fahrenheit) above historical averages. Even a massive reduction in emissions now won’t help reduce what is already there, because some carbon dioxide stays in the atmosphere for thousands of years.

When the oceans take in more of the excess, that leads to another problem.

Oceans become too acidic

Ocean acidity pH of Hawaii waters 8.15 8.11 8.10 8.05 8.07 8.00 2012 1988 Source: Hawaii Ocean Time-series/NSF

Oceans absorb more than a quarter of the carbon dioxide humans produce every year. When the carbon dioxide dissolves, it reacts with water and becomes carbonic acid. The extra carbon dioxide is changing the ocean’s chemistry.

What does this gauge show?

Acidity is expressed on the somewhat confusing pH scale, where smaller numbers are more acidic. The ocean’s pH has dropped from a preindustrial level of 8.21 to less than 8.1, according to NOAA. That small-looking change means the ocean’s acidity has increased by about 30 percent.

What’s the problem?

All that carbonic acid gobbles carbonate, leaving less for sea creatures who need it to make shells, skeletons and coral reefs. Among the vital animals harmed are tiny plankton, a key part of the food web.

[Climate-change warnings include rising seas and wild weather shifts. But giant flying boulders?]

Methane adds to warming

Atmospheric methane In parts per billion 1,900 1,860 1,800 1,700 1,698 1,600 2014 1988 Source: NOAA Earth System Research Lab.

Methane, another greenhouse gas, comes from natural sources such as livestock digestion, wetlands and the melting of permafrost. But some of the same industries that spew carbon dioxide release a lot of methane as well. Other big methane producers are landfills and agriculture — especially rice cultivation.

What does this gauge show?

In 2014, methane in the atmosphere was measured at more than two and a half times the preindustrial level.

What’s the problem?

Methane is a much smaller and shorter-lasting part of the atmosphere than carbon dioxide, but it is much more efficient at trapping heat.

Earth’s surface heats up

Global surface temperature In degrees centigrade, compared with 1951-1980 average 0.75 0.6 0.4 0.2 0 –0.2 –0.4 2014 1880 Source: NASA/GISS

We all know that some years are warmer and some are colder, thanks in part to global weather patterns such as El Niño and La Niña. But the trend has shot upward since reliable record-keeping began in 1880. The 10 warmest years on record have all occurred since 1998.

What does this gauge show?

As of September, the world’s annual average was just over one degree Celsius (1.8 degrees Fahrenheit) warmer than preindustrial levels, according to the World Meteorological Organization. That puts the world halfway to the danger zone of two degrees Celsius (3.6 degrees Fahrenheit) warmer than historical averages.

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What’s the problem?

Many climate scientists believe warming that exceeds two degrees Celsius will trigger enormous consequences, greater than those we are already seeing. So far, 2015 is the hottest year on record, and many parts of the world have experienced record-breaking heat. Even the good news is bad: The record cold spot in the North Atlantic may have been the result of Greenland’s melting ice sheet.

C oldest W armest N O D A T A on r eco r d on r eco r d Source: National Oceanographic and Atmospheric Administration Cooler than average Warmer than average N O D A T A C oldest W armest on r eco r d on r eco r d Source: National Oceanographic and Atmospheric Administration

Rising temperatures don’t just make you sweat; they affect weather patterns. Heat waves are longer and stronger. Warmer air holds more moisture, so rainstorms drop more water. But the temperature rise also disrupts animal habitats and even alters the spread of disease. For instance, deer ticks, which carry Lyme disease and other illnesses, are projected to spread much farther north as temperatures rise.

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PROBABILITY OF TICK PRESENCE 0 20 40 60 80 100% CANADA UNITED STATES The black-legged tick, or deer tick, transmits Lyme disease, babesiosis, Powassan virus, Borrelia anaplasmosis. Sources: EcoHealth, CDC PROBABILITY OF TICK PRESENCE 0 20 40 60 80% CANADA UNITED STATES The black-legged tick, or deer tick, transmits Lyme disease, babesiosis, Powassan virus, Borrelia anaplasmosis. Sources: EcoHealth, Centers for Disease Control and Prevention

Ocean temperatures rise as well

Ocean heat As 1022-joule deviation from 1971 to 2000 average 13.0 12 9 6 3 0 –3 –6 2014 1955 Source: NOAA, CSIRO, MRI/JMA

What does this gauge show?

Between 1880 and 2014, the surface temperature of the ocean rose at an average of about 0.1 degree Fahrenheit per decade, according to Climate Central. Our scale represents the heat stored in the ocean rather than just the surface temperature, and it is expressed in joules, a unit of stored heat energy. The takeaway is that the ocean has gotten steadily warmer since the 1950s.

What’s the problem?

For starters, water expands as it gets warmer, adding to the rising sea level. But the effects go far beyond that. Warmer water absorbs less carbon dioxide, so the warmer the ocean gets, the less it will be able to help clear the atmosphere. Ocean species such as crabs and fish follow cooler water toward the poles, changing the way for life of people who depend on them. Hurricanes may, on average, become stronger because warm water is their chief energy source. Warm water also contributes to melting ice.

Ice sheets are melting

Land ice In hundreds of gigatons of ice Relative to time series average 14.9 15 Greenland 10 9.7 5 Antarctica 0 2014 2002 –5 –10 –10.1 –15 Source: NASA Jet Propulsion Laboratory –20 –19.1

Earth’s two massive ice sheets are disappearing because of an atmospheric double whammy. In Greenland, the mechanics are fairly clear: Warmer air melts ice on top, and warmer water brought in by changing ocean currents melts submerged parts of glaciers. Scientists are less sure about what is going on in Antarctica. They think that a wind phenomenon is bringing a sub-sea layer of warm water closer to the continent, with that water eating away at the gigantic glaciers. No one has proved a link to global warming, but many find it hard to believe it could be a coincidence.

What do these gauges show?

Nearly every study has concluded that the massive ice sheets at the poles are melting, but measuring the decline is difficult and estimates vary. According to satellite data from NASA’s Jet Propulsion Laboratory, the Greenland ice sheet has lost 287 billion metric tons per year since 2002; Antarctica has lost about 134 billion metric tons per year.

Areas losing a Warming Ice thickness of sea beds shelves 10 to 23 feet of ice per year EAST WEST ANTARCTICA ANTARCTICA Sources: Science, NOAA, NASA, Nature, AntarcticGlaciers.org Areas losing a thickness Warming Ice shelves of 10 to 23 feet of ice per year sea beds WEST EAST ANTARCTICA ANTARCTICA Sources: Science, NOAA, NASA, Nature, AntarcticGlaciers.org

What’s the problem?

The obvious answer is that all the extra water causes sea levels to rise, but that’s not the only issue. A large influx of fresh meltwater pouring into the oceans could change ocean currents that circulate around the the globe. For instance, the cold spot in the North Atlantic could alter the powerful Gulf Stream that carries critical warmth to Northern Europe.

Gr eenland N O R TH EUROPE AME R ICA Atlantic Ocean AF R ICA P aciﬁc Ocean SOUTH AME R ICA AN T A R CTICA Source: U.S. Geological Survey, NOAA r w Gr eenland N O R TH EUROPE AME R ICA Atlantic Ocean AF R ICA SOUTH Indian AME R ICA P aciﬁc Ocean Ocean Southern Ocean AN T A R CTICA Source: U.S. Geological Survey, NOAA

Inland glaciers are melting, too, endangering plants and animals that live at the edges of the ice and in the cold streams around it. Healthy glaciers also replenish water supplies that humans rely on. At Montana’s Glacier National Park, for instance, only 25 glaciers remain out of the 150 that flowed through the park in the middle of the 19th century.

[As temperatures rise, many American glaciers could vanish in a few decades]

Northern Rocky Mountain Science Center

More obvious — and easier to measure — than melting land ice is melting sea ice.

Arctic sea ice is disappearing

Arctic sea ice minimum In millions of square kilometers 6 5.95 4.63 4 2 0 2015 2002 Source: National Snow and Ice Data Center

Ice in Arctic seas melts some every summer and refreezes every winter. In recent years, however, more and more is melting and less and less is refreezing. Ice coverage is more than a third less than it was 30 years ago.

What does this gauge show?

Since satellites began to observe the Arctic sea ice in 1979, the sea ice minimum — meaning the amount of ice left at the time of peak melt in September — has been declining at 13.3 percent per decade. Some scientists predict that within a few decades, the Arctic may have no ice at all in summer.

[As temperatures rise, many American glaciers could vanish in a few decades]

What’s the problem?

Melting sea ice creates a downward spiral. White ice reflects the sun’s rays, so when the ice is gone, the dark water absorbs the sun’s heat and gets warmer, causing more ice to melt. This disrupts the habitat, feeding and migration patterns of Arctic animals such as whales, walruses and polar bears. Many climatologists also believe Arctic ice — or the lack of it — will alter global weather patterns.

Sea levels creep higher and higher

Global mean sea level Change in millimeters relative to Jan. 1993 83.47 80 60 40 20 0 2015 1993 Source: NASA Goddard Space Flight Center

Here’s where the water meets the road: Melting ice raises sea levels, and so does heat, because warmer water naturally expands. Scientists say that the last time the carbon dioxide in the atmosphere was this high, seas were significantly higher than they are now — estimates range from less than 10 feet to more than 100.

What does this gauge show?

Worldwide, sea level is more than three inches (83 millimeters) higher than it was in 1993, according to NASA’s Goddard Space Flight Center, which measures sea level using lasers. Many scientists predict that levels will rise an additional two to four feet by the end of this century.

What’s the problem?

Clearly, higher water means the potential for massive flooding, not just for coastlines but for people who live on low-lying land — which means about a third of the U.S. population. Higher sea level also means storm surges can roll farther inland, a growing danger when hurricanes are predicted to become more violent. And saltwater intrusion into fresh water can upset fragile ecosystems and economies that depend on them, such as shrimping and crabbing.

“There are so many things we’re locking into our system that are going to take thousands of years to play out,” said Bernadette Woods Placky, chief meteorologist for Climate Central, “and they just get more dramatic as they go along.”