What is climate change? The mechanism, the evidence, and the open questions

Climate change refers to the long-term shift in Earth's energy balance driven primarily — over the last 150 years — by changes in atmospheric composition. The mechanism is well-understood, the evidence is overwhelming, and the genuinely open questions are different from the ones that public discourse usually emphasizes.

This article walks through all three.

The mechanism

Earth receives roughly constant energy from the sun. To stay at a stable temperature, it must radiate the same amount of energy back to space. Greenhouse gases — water vapor, carbon dioxide, methane, nitrous oxide, others — selectively absorb infrared radiation in the wavelengths that would otherwise escape. The energy doesn't disappear; it's re-emitted in all directions, including back toward the surface. The result is a warmer surface than would otherwise be in equilibrium with incoming sunlight (NASA Science: Climate Change Evidence).

This effect is not contested. It was characterized in the 19th century, has been measured in laboratories for over a century, and is the only physical explanation consistent with the observed warming pattern. The IPCC's Sixth Assessment Report describes the human attribution of recent warming as "unequivocal." The greenhouse effect is also why Earth is habitable — without it, the planet would be roughly 33°C colder. The question has never been whether greenhouse gases warm the planet. The question is what happens when their concentration changes.

Atmospheric carbon dioxide has risen from a stable pre-industrial value of ~280 ppm to roughly 425 ppm today. This is the highest concentration in at least 800,000 years (the limit of direct ice-core measurement) and almost certainly the highest in several million years. The isotopic signature of the added carbon is unambiguously fossil — it carries the depleted carbon-14 fraction that fossil fuels have because they have been buried long enough for radioactive decay.

The evidence

The case for current warming and its human cause does not rest on any single line of evidence. It rests on the convergence of several independent ones, each of which would be sufficient on its own.

Direct measurement. The instrumental temperature record now spans more than 150 years, with multiple independent groups producing reconstructions from raw station data. They agree on the magnitude (about 1.2°C above pre-industrial) and on the acceleration over the last 50 years.

Spatial pattern. Greenhouse warming has a distinctive fingerprint: more warming over land than ocean, more at high latitudes than low, more at night than day, more in winter than summer. Solar warming has the opposite pattern in several of these. The observed pattern matches greenhouse warming, not solar.

Vertical structure. Greenhouse warming heats the lower atmosphere (troposphere) and cools the upper atmosphere (stratosphere). Solar warming would heat both. The observed structure matches greenhouse warming.

Paleoclimate. Reconstructions from ice cores, ocean sediments, and tree rings show that the last 50 years are warmer than any comparable period in at least 1,000 years, and that the rate of warming exceeds anything in the Holocene record.

Sea level. Sea level is rising at a rate consistent with thermal expansion of the warming ocean plus measured ice-sheet and glacier loss. The two budgets close — there is no missing source.

Energy budget. The Earth's net energy imbalance is now directly measurable from satellites. It matches the imbalance predicted by greenhouse theory.

These are independent. Every one of them would have to be wrong, in a coordinated way, for the conclusion to fail.

What is genuinely uncertain

Public discussion often suggests that the central uncertainty is whether climate change is happening or whether humans are causing it. Working climate scientists almost never argue about either. The genuinely open questions are different.

Regional precipitation. Global mean precipitation responds robustly to warming, but regional patterns depend on circulation changes that current models do not yet resolve to confidence. Whether a specific region will get wetter, drier, or seasonally variable is harder to predict than mean temperature.

Cloud feedback. Clouds can amplify or damp warming depending on their type and altitude. The net cloud feedback is the largest single source of uncertainty in equilibrium climate sensitivity. Recent observations have narrowed the range, but it remains the dominant uncertainty in long-range projections.

Ice-sheet dynamics. The pace of West Antarctic and Greenland ice loss depends on processes — basal melt, ice-shelf collapse, marine ice-cliff instability — that are still being characterized. The next century's sea-level rise could plausibly span a meter of difference depending on which mechanisms engage.

Carbon-cycle feedbacks. Land and ocean currently absorb about half of human carbon emissions. Whether this absorption persists, weakens, or reverses under continued warming is the largest single uncertainty in the trajectory of atmospheric CO₂ at any given emissions path.

Tipping points and reversibility. Several large Earth-system components — Amazon dieback, Arctic methane release, AMOC slowdown — could reorganize abruptly if warming exceeds threshold values. The threshold values themselves are uncertain.

These uncertainties are not symmetric. Most of them, on the central estimate, point toward worse outcomes than the consensus median, not better.

The takeaway

Climate change is a thermodynamic consequence of changing atmospheric composition. The mechanism is undisputed; the attribution to human activity is among the most robust empirical findings in modern science. The active research frontier is on magnitude, regional impact, feedback strength, and reversibility — not on the foundational claims.

When you encounter a climate argument, the relevant filter is: which of those frontier questions is it addressing? Most useful disagreements live there. The disagreements that don't — the ones that re-litigate the basic mechanism or the attribution — have been settled long enough that engaging them is no longer where the science is.

Sources

1. IPCC — Climate Change 2021: The Physical Science Basis (AR6 Working Group I), Intergovernmental Panel on Climate Change, 2021. The authoritative synthesis of current evidence on climate mechanisms, attribution, and projections.
2. IPCC — AR6 Synthesis Report: Climate Change 2023, Intergovernmental Panel on Climate Change, 2023. Integrates findings across all three working groups.
3. NASA — Climate Change: Evidence, NASA Science, ongoing. Plain-language summary of the multiple independent lines of evidence for warming.
4. NOAA — Climate.gov: Science & Information for a Climate-Smart Nation, National Oceanic and Atmospheric Administration. Authoritative observational data and explainers.
5. WHO — Climate change and health (fact sheet), World Health Organization, October 2023. Health-impact framing of the scientific consensus.
6. EEA — Climate change in depth, European Environment Agency. European observational record and policy context.