Causes of climate change

Based on Wikipedia: Causes of climate change

The Detective Story of Modern Climate Science

Here's a question that has occupied thousands of scientists for decades: Is the warming we're seeing on Earth actually caused by humans, or is it just nature doing what nature does?

After all, the planet has warmed and cooled many times before. Ice ages have come and gone. So what makes scientists so confident—and I mean genuinely confident, not politically motivated confident—that this time is different?

The answer lies in fingerprints.

The Greenhouse Effect: A Blanket Made of Gas

To understand what's happening, you need to grasp one fundamental concept: the greenhouse effect. And despite its name, it has almost nothing to do with the glass buildings where people grow tomatoes.

Sunlight passes through Earth's atmosphere without much trouble. The atmosphere is essentially transparent to visible light. That sunlight hits the ground, warms it up, and the ground radiates that energy back out—but as heat, not light. Heat is infrared radiation, which has a longer wavelength than visible light.

Here's where it gets interesting. Certain gases in the atmosphere—carbon dioxide, methane, water vapor, and a few others—are transparent to visible light but opaque to infrared radiation. They let the sunlight in, but they trap the heat trying to escape. It's like wearing a jacket that lets the wind through but keeps your body heat close.

Without any greenhouse effect at all, Earth would be a frozen rock. The average temperature would be something like negative eighteen degrees Celsius. Life as we know it wouldn't exist. So the greenhouse effect isn't inherently bad—it's essential.

The problem arises when you add more of these heat-trapping gases to the atmosphere.

The Keeling Curve: A Single Scientist's Gift to Humanity

In 1958, a scientist named Charles David Keeling started doing something remarkably simple: measuring the concentration of carbon dioxide in the atmosphere with extreme precision. He set up his instruments on Mauna Loa in Hawaii, far from factories and cities, where the air would give him a clean global average.

His first measurement showed 313 parts per million—meaning that out of every million molecules of air, 313 of them were carbon dioxide.

That number has become one of the most important datasets in all of science. It's called the Keeling Curve, and it shows an unmistakable upward trend, year after year after year. By May 2019, it had reached 415 parts per million. Without human activity, it would be around 280 parts per million—roughly where it sat before the Industrial Revolution.

To put this in perspective: the last time carbon dioxide levels were this high was between 2.6 and 5.3 million years ago. There were no humans. There were barely any recognizable mammals. The world was a fundamentally different place.

And in 2022 through 2024, according to the National Oceanic and Atmospheric Administration, the concentration increased faster than ever before. In November 2025, the Global Carbon Budget predicted that carbon dioxide emissions from burning coal, oil, and gas would hit a record 38.1 billion tonnes—up 1.1 percent from the prior year.

Where Does All This Carbon Come From?

The short answer: fossil fuels.

Coal, oil, and natural gas are called fossil fuels because they're made from ancient organisms—plants and animals that died hundreds of millions of years ago, got buried under sediment, and were slowly transformed by heat and pressure into energy-dense carbon compounds. When you burn them, you're releasing carbon that was locked away underground for geological time periods.

The breakdown of global emissions is stark. In 2019, humanity released greenhouse gases equivalent to 59 billion tonnes of carbon dioxide. Of that total, 75 percent was carbon dioxide itself, 18 percent was methane, 4 percent was nitrous oxide, and 2 percent was various fluorinated gases—entirely artificial compounds that don't exist in nature.

Carbon dioxide comes from burning fossil fuels for transportation, manufacturing, heating, and electricity. But it also comes from deforestation (when you cut down a tree, the carbon stored in its wood eventually returns to the atmosphere) and from industrial processes. Making cement, for instance, involves a chemical reaction that releases carbon dioxide as a byproduct. Same with making steel, aluminum, and fertilizer.

Methane, the second-biggest contributor, comes from stranger sources. Livestock produce it through digestion—the technical term is enteric fermentation, which is a polite way of saying that cows burp and fart methane. Rice paddies release it. Landfills release it as garbage decomposes. Coal mines leak it. Oil and gas operations vent it.

Nitrous oxide—you might know it as laughing gas from the dentist's office—comes largely from fertilizers. When nitrogen-based fertilizers break down in soil, microbes convert some of that nitrogen into nitrous oxide.

The Numbers That Changed Everything

By 2019, carbon dioxide concentrations had increased by about 48 percent compared to 1750. Methane had increased by 160 percent. These aren't subtle changes. These are dramatic alterations to the chemical composition of our atmosphere.

The result has been measurable warming. Between 2010 and 2019, compared to the period from 1850 to 1900, the planet warmed by somewhere between 0.9 and 1.2 degrees Celsius. The best estimate of how much of that warming was caused by humans? Between 0.8 and 1.3 degrees Celsius, with a most likely value of 1.07 degrees.

Read those numbers again. The observed warming is almost entirely explained by human activity. Natural factors—changes in the sun, volcanic eruptions, natural climate cycles—account for only about plus or minus 0.1 degrees Celsius. Natural variability adds another plus or minus 0.2 degrees. Everything else is us.

How Do Scientists Know It's Not the Sun?

This is where the fingerprinting gets clever.

Different causes of climate change leave different signatures. If the sun were responsible for the warming, you'd expect the entire atmosphere to warm, from the surface all the way up. More solar energy coming in would heat everything.

But that's not what we observe.

What we actually see is the lower atmosphere (the troposphere, where we live and where weather happens) getting warmer, while the upper atmosphere (the stratosphere) is actually cooling. This is exactly what you'd expect if the warming were caused by greenhouse gases trapping heat near the surface rather than by extra energy coming in from above.

Think about it this way: if the greenhouse effect is strengthening, more heat is being absorbed before it can escape to the upper atmosphere. The lower atmosphere gets warmer; the stratosphere gets less heat and cools down. It's an unmistakable fingerprint.

Scientists have also compared observations to climate models. When they run models that include only natural factors—solar variations, volcanic eruptions, orbital changes—the models can't replicate the observed warming. The numbers don't work. But when they add human greenhouse gas emissions to the models, suddenly the match is nearly perfect.

The Logarithmic Twist

There's an interesting complication in the physics. The warming effect of greenhouse gases follows a logarithmic relationship, not a linear one. This means that each additional molecule of carbon dioxide has a slightly smaller warming effect than the molecule before it.

You might think this is good news—that we could keep emitting and eventually the effect would taper off. But two other factors complicate this picture.

First, only about half of the carbon dioxide we emit actually stays in the atmosphere. The rest gets absorbed relatively quickly by what scientists call carbon sinks—primarily the oceans and forests. Trees absorb carbon dioxide through photosynthesis. Oceans absorb it directly from the air. These sinks have been doing humanity an enormous favor by soaking up a huge portion of our emissions.

But as temperatures rise, carbon sinks become less efficient. A warmer ocean holds less dissolved gas. Stressed forests absorb less carbon or even start releasing it. So while each unit of carbon dioxide has a diminishing direct warming effect, we're also losing our natural buffers at the same time.

Second, there are feedback loops. Warming causes other changes that cause more warming. Water vapor is the biggest example. A warmer atmosphere holds more water vapor. Water vapor is itself a greenhouse gas. So warming leads to more water vapor leads to more warming. This is a positive feedback—not positive in the sense of being good, but positive in the sense of amplifying the initial change.

These two effects—diminishing direct warming and increasing feedbacks—roughly cancel each other out. The practical result is that warming increases more or less linearly with total cumulative emissions. Every additional ton of carbon dioxide we emit adds roughly the same amount of warming to the system.

The Masking Effect We're Losing

Here's something that might surprise you: some of our pollution has actually been cooling the planet.

When we burn coal, especially dirty coal with high sulfur content, we release sulfur dioxide into the atmosphere. This sulfur dioxide forms tiny particles called aerosols that reflect sunlight back into space. They also change the properties of clouds, making them brighter and more reflective.

From about 1961 to 1990, scientists observed a phenomenon they called global dimming—less sunlight was reaching Earth's surface. This wasn't because the sun was getting weaker; it was because aerosol pollution was reflecting more sunlight away.

In a sense, air pollution was partially canceling out greenhouse warming. The planet wasn't warming as fast as the greenhouse gases alone would have caused because the aerosols were providing a cooling counterforce.

But here's the problem: aerosol pollution is genuinely harmful. It causes acid rain. It damages lungs. It kills millions of people every year. So for very good reasons, countries have been working to reduce sulfur emissions and clean up particulate pollution.

This cleanup is succeeding. And as the aerosols diminish, the full force of the underlying greenhouse warming is being unmasked. We're effectively losing a protective, if toxic, shield.

The Special Case of Black Carbon

Not all aerosols cool the planet. Black carbon—essentially soot—actually contributes to warming. It's dark, so it absorbs sunlight rather than reflecting it. And when black carbon lands on snow or ice, the effects are particularly severe.

Fresh snow is one of the most reflective surfaces on Earth. It bounces most sunlight back into space. But when soot settles on snow, it darkens the surface. The darkened snow absorbs more energy, warms up, and melts faster.

This matters enormously in places like the Arctic. Scientists estimate that limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 degrees Celsius by 2050. That's a significant amount from addressing a single factor.

The Land Use Story

Climate change isn't only about burning fossil fuels. How we use land matters too.

According to the Food and Agriculture Organization, about 30 percent of Earth's land is essentially unusable for humans—glaciers, deserts, the most extreme environments. Of the rest, 26 percent is forest, 10 percent is shrubland, and 34 percent is agricultural land.

Between 1750 and 2007, roughly one-third of all human carbon dioxide emissions came from land use changes—primarily from the conversion of forests to agricultural land. When you clear a forest, the stored carbon is released. And you've also removed a carbon sink that would have continued absorbing carbon dioxide from the atmosphere for decades or centuries.

The drivers of deforestation are varied. Between 2001 and 2018, about 27 percent was permanent clearing for crops and livestock. Another 24 percent was temporary clearing for shifting cultivation—the practice of clearing land, farming it until the soil is depleted, then moving on. About 26 percent was logging for wood products. And 23 percent was due to wildfires, many of which are becoming more severe because of climate change itself.

Forests also affect climate through mechanisms beyond carbon storage. They influence how much sunlight gets reflected versus absorbed. A dark forest absorbs more solar energy than bright grassland or snow-covered fields. They affect wind patterns and cloud formation. In tropical and temperate regions, forests generally have a net cooling effect. Closer to the poles, the picture is more complicated—a snow-covered field might actually be cooler than a dark forest that absorbs more sunlight.

The Livestock Factor

More than 18 percent of human greenhouse gas emissions are attributed to livestock and related activities.

This might seem surprising. Cows don't burn fossil fuels. But they do digest food in a way that produces methane—a lot of methane. Their manure produces more. The fertilizer used to grow their feed releases nitrous oxide. The land cleared to graze them or grow their feed releases stored carbon.

The numbers break down like this: livestock contribute about 9 percent of global carbon dioxide emissions (mainly through land use changes), 35 to 40 percent of methane emissions, and 64 percent of nitrous oxide emissions.

Methane deserves special attention. Molecule for molecule, it's far more potent as a greenhouse gas than carbon dioxide. The good news is that methane breaks down in the atmosphere relatively quickly—over years to decades rather than centuries. The bad news is that we keep adding more of it.

The Energy Imbalance

There's a simple way to understand what's happening to Earth's climate: measure how much energy comes in from the sun versus how much radiates back out into space. The difference is the energy imbalance.

If Earth is absorbing more energy than it's releasing, it will warm up. If it's releasing more than it absorbs, it will cool down. In equilibrium, these would be equal.

Observations from space show that Earth's energy imbalance has been growing. By 2023, it had reached values twice as high as the best estimate from the Intergovernmental Panel on Climate Change. The planet is absorbing significantly more energy than it's radiating away.

All that excess energy has to go somewhere. Most of it goes into the oceans, which is why they've been warming dramatically. Some goes into melting ice. Some warms the atmosphere. Some warms the land. But it doesn't disappear. The physics of energy conservation guarantees that if there's an imbalance, something must change.

Natural Variability and Why It's Not the Explanation

The climate system has natural variability even without any external forcing. The most famous example is the El Niño–Southern Oscillation—a cycle of warming and cooling in the tropical Pacific that affects weather patterns worldwide. There are other oscillations and cycles too.

Some climate skeptics have argued that the observed warming could just be natural variability. Maybe we're just seeing a random warm period that will eventually swing back to cooler temperatures.

But this argument doesn't survive contact with the data. When scientists study historical climate changes—using ice cores, tree rings, coral records, and other proxies—they find that the recent changes in global surface temperature are unusual. The rate of warming is unusual. The global synchronization of warming is unusual.

Natural variability can explain fluctuations of a few tenths of a degree over decades. It cannot explain a steady, century-long warming trend that accelerates over time and matches precisely with rising greenhouse gas concentrations.

The Scientific Consensus

The scientific community has concluded—not by vote or by authority, but by the accumulation of evidence from thousands of studies—that it is, in the words of the Intergovernmental Panel on Climate Change, "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times."

That word "unequivocal" is important. Scientists are professionally cautious people. They deal in probabilities and uncertainties. They hedge their conclusions. For them to use the word "unequivocal" means they've reached the point where there's no reasonable doubt remaining.

Around 200 scientific organizations worldwide support this consensus. Not because they all got together and agreed to say the same thing, but because they've all independently reviewed the evidence and reached the same conclusion.

The evidence comes from four main lines:

First, we understand the physics. Greenhouse gases trap heat. We can measure this in laboratories. We can calculate the warming effect from first principles.

Second, we have records of past climate. They show that current changes are happening faster and more uniformly than anything in the historical or geological record that wasn't caused by a major catastrophe.

Third, our climate models only match observations when we include human greenhouse gas emissions. Models with only natural factors can't explain what we're seeing.

Fourth, we've ruled out the alternatives. Solar variation doesn't match the pattern. Volcanic activity doesn't match the pattern. Natural oscillations don't match the pattern. Only greenhouse warming fits the fingerprint.

What This Means Going Forward

The physics of greenhouse warming is now well understood. Every additional unit of carbon dioxide we emit adds roughly the same amount of warming. The carbon sinks are weakening. The masking effect of aerosols is fading. The feedback loops are amplifying.

This isn't a prediction about the future so much as an observation about the present. The warming is already happening. The question is how much additional warming we'll add to what's already locked in.

The relationship between emissions and warming is essentially linear. If we emit more, we get more warming. If we emit less, we get less additional warming. There's no threshold we've crossed that makes further action pointless. There's no level of emissions that's safe. Every fraction of a degree matters.

The science here is clear. What to do about it involves economics, politics, ethics, and technology—domains where reasonable people can and do disagree. But on the basic question of causation, the detective work is done. The fingerprints are on the weapon. The evidence is in.