What Is Climate Change? The Science Behind a Warming World

The Greenhouse Effect: Why Earth Isn't Frozen

Without any greenhouse effect at all, Earth's average surface temperature would be approximately -18°C (0°F) — a frozen, lifeless world. The natural greenhouse effect keeps Earth at a habitable +15°C (59°F) by trapping some of the Sun's energy in the atmosphere.

Here's how it works: sunlight passes through the atmosphere and warms Earth's surface. The surface radiates this energy back as infrared radiation (heat). Greenhouse gases — primarily water vapor, carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) — absorb some of this outgoing infrared radiation and re-radiate it in all directions, including back toward the surface. This creates an insulating effect.

The natural greenhouse effect is essential for life. The problem is that human activities are intensifying it by adding greenhouse gases far beyond natural levels. Since the Industrial Revolution began around 1750, atmospheric CO₂ has risen from approximately 280 parts per million (ppm) to over 420 ppm in 2024 — a 50% increase. We know this through direct measurements at Mauna Loa Observatory (since 1958) and ice core records going back 800,000 years.

CO₂ has not been this high in at least 4 million years. During the entire span of human civilization — the last 10,000 years — CO₂ levels fluctuated naturally between about 260-280 ppm. The current concentration is unprecedented in human history, and the rate of increase (about 2.5 ppm per year) is at least 10 times faster than any natural change in the geological record.

What's Actually Happening: The Data

Earth's average surface temperature has risen by approximately 1.1°C (2.0°F) since 1880, with most of that warming occurring since 1975. The ten warmest years in recorded history have all occurred since 2010. The year 2023 was the warmest on record, and 2024 was on track to match or exceed it.

This might sound trivially small — 1.1 degrees — but global averages mask enormous regional and seasonal variations. The Arctic has warmed roughly 3-4 times faster than the global average, with Arctic sea ice declining by about 13% per decade since satellite measurements began in 1979. In September 2023, Arctic sea ice extent was 1.6 million square kilometers below the 1981-2010 average.

Sea levels have risen approximately 20 centimeters since 1900 and are currently rising at 3.6 millimeters per year — accelerating. The rate has doubled since the 1990s. This is driven by two factors: thermal expansion (warmer water takes up more volume) and ice sheet melting, particularly in Greenland and West Antarctica. Greenland alone is losing approximately 270 billion tonnes of ice per year.

Extreme weather events are increasing in frequency and intensity. A 2021 study in Nature Climate Change found that extreme heat events that previously occurred once every 50 years now occur roughly every 10 years. The 2023 Canadian wildfires burned over 18 million hectares — more than twice the previous record — sending smoke across the Atlantic to Europe.

Ocean acidification is the 'other CO₂ problem.' About 30% of emitted CO₂ is absorbed by the oceans, where it forms carbonic acid. Ocean pH has decreased by 0.1 units since pre-industrial times (a 26% increase in acidity), threatening coral reefs, shellfish, and the entire marine food chain.

Who's Responsible: Sources of Emissions

Global greenhouse gas emissions reached approximately 57.4 billion tonnes of CO₂ equivalent in 2022 (UNEP Emissions Gap Report). The major sources break down as follows:

Energy production (electricity and heat) accounts for about 25% of global emissions. Burning coal, natural gas, and oil for electricity generation remains the single largest source. Coal is the dirtiest fuel, producing roughly twice the CO₂ per unit of energy as natural gas.

Transportation contributes approximately 16% of global emissions. Road vehicles (cars, trucks, buses) dominate, with aviation accounting for about 2.5% and shipping about 2.5%. The average American car emits approximately 4.6 metric tons of CO₂ per year.

Industry accounts for about 21% — cement production (8% of global CO₂ on its own), steel manufacturing, chemicals, and other industrial processes that require intense heat or chemical reactions that release CO₂.

Agriculture and land use contribute roughly 22% when including deforestation. Cattle produce methane through digestion (enteric fermentation), rice paddies emit methane from waterlogged conditions, and nitrogen fertilizers release nitrous oxide. Deforestation — primarily in the Amazon, Congo Basin, and Southeast Asia — releases stored carbon and eliminates carbon sinks.

Buildings account for about 6% through heating, cooling, and cooking.

By country, China is the largest annual emitter (roughly 30% of global emissions), followed by the United States (14%), India (7%), the EU (7%), and Russia (5%). However, per capita, Americans emit about 15 tonnes of CO₂ per person per year — roughly twice the Chinese average and eight times the Indian average.

What 1.5°C and 2°C of Warming Mean

The 2015 Paris Agreement set the goal of limiting warming to 'well below 2°C above pre-industrial levels' while 'pursuing efforts to limit the temperature increase to 1.5°C.' These aren't arbitrary numbers — they represent thresholds beyond which certain impacts become dramatically worse.

The IPCC's 2018 special report found that the difference between 1.5°C and 2°C is enormous. At 1.5°C, 70-90% of coral reefs die. At 2°C, virtually all (99%) are lost. Sea level rise by 2100 would be about 10 centimeters less at 1.5°C versus 2°C — the difference affecting approximately 10 million additional people living in coastal areas. The probability of an ice-free Arctic summer increases from once per century (1.5°C) to once per decade (2°C).

To limit warming to 1.5°C, global CO₂ emissions need to fall by approximately 45% from 2010 levels by 2030 and reach net-zero by 2050. Current policies put the world on track for approximately 2.5-2.9°C of warming by 2100. The gap between commitments and trajectories remains large.

Some impacts are already locked in regardless of future emissions. Even if all emissions stopped today, the CO₂ already in the atmosphere would continue warming the planet for decades. Sea levels would continue rising for centuries due to the thermal inertia of the oceans. The question isn't whether climate change will happen — it's happening now — but how severe it will become.

Sources: IPCC Sixth Assessment Report (2021-2023), NASA GISS Temperature Analysis, NOAA Arctic Report Card, UNEP Emissions Gap Report 2023, Carbon Brief analysis.