What the evidence says about global warming — part one
- Editorial Team SDG13
- Jul 20
- 7 min read
Updated: Jul 28
Published on 20 July 2026 at 02:00 GMT
By Editorial Team SDG13
Few scientific subjects have moved as decisively from specialist research into public debate as global warming. Governments, businesses, universities and civil society now discuss its causes and consequences almost daily. Yet the intensity of that discussion has sometimes obscured a more fundamental question: how real is global warming when judged not by political positions, computer projections or isolated weather events, but by direct observation?
The answer begins with an important distinction. Weather describes atmospheric conditions over hours, days or seasons. Climate describes patterns measured over much longer periods. A heatwave cannot, by itself, prove global warming, just as an unusually cold winter cannot disprove it. Climate science depends on trends: repeated measurements collected over decades and compared across independent observing systems.
Those systems now extend across almost every component of the planet. Thermometers record conditions at thousands of land stations. Ships, buoys and autonomous floats measure the oceans. Weather balloons sample the atmosphere at different altitudes. Satellites observe changes across land, sea and ice. Paleoclimate evidence preserved in tree rings, sediments and ice cores provides a longer historical context.
Each method has limitations. Urban development may influence individual weather stations. Satellite instruments require careful calibration. Ocean measurements have changed as technologies have improved. Historical records contain gaps and must be analysed with appropriate statistical methods.
Yet the scientific strength of these observations does not depend on any one instrument or organisation. It lies in their convergence. Independent datasets produced by different institutions, using different methods, describe broadly the same long-term development: the Earth has warmed substantially since the late nineteenth century, and the rate of warming has been particularly pronounced during recent decades.
The Sixth Assessment Report of the Intergovernmental Panel on Climate Change concluded that human influence has unequivocally warmed the atmosphere, ocean and land. This assessment does not rest on a vote or a single model. It draws upon a large body of observations, physical understanding and peer-reviewed research accumulated across generations.
Why a global average matters
Temperature changes experienced in everyday life can make the figures associated with global warming seem deceptively small. A difference of one degree between one afternoon and the next is barely remarkable. A change of approximately one degree in the global average, however, represents something very different.
Global mean temperature describes the condition of an enormous physical system encompassing the atmosphere, land surface and oceans. Raising that average requires the accumulation of a vast quantity of additional energy. It does not mean that every location warms equally or that every year is warmer than the one before it. Regional conditions continue to vary, and natural processes such as El Niño, La Niña, volcanic eruptions and changes in ocean circulation still influence individual years.
The long-term trend nevertheless remains visible beneath that variability.
The IPCC estimated that global surface temperature during 2011–2020 was approximately 1.1°C above the 1850–1900 average. More recent annual temperatures have risen higher. The World Meteorological Organization calculated that 2024 was approximately 1.55°C, with an uncertainty range of about ±0.13°C, above the 1850–1900 baseline. It was probably the first complete calendar year to exceed 1.5°C above that reference period.
That result requires careful interpretation. The Paris Agreement’s 1.5°C objective concerns sustained long-term warming, not a temporary exceedance during one year. Natural variability can push an individual year above or below the underlying trend. A single year at approximately 1.55°C therefore does not, on its own, establish that the long-term threshold has been permanently crossed.
The distinction is not an attempt to minimise the figure. It is an example of the precision climate science requires. Annual anomalies, decadal averages and long-term warming estimates measure related but different things and should not be used interchangeably.
The subsequent year remained exceptionally warm. According to the WMO’s consolidated assessment, 2025 was approximately 1.44°C above the 1850–1900 average and ranked as either the second- or third-warmest year in the principal datasets examined. The years from 2015 to 2025 were the eleven warmest in the instrumental record.
A small number with planetary consequences
A useful historical comparison illustrates why changes in the global average matter.
During the Last Glacial Maximum, approximately 20,000 years ago, global mean temperature is estimated to have been roughly 5–6°C lower than today. That difference was associated with enormous ice sheets covering large parts of North America and northern Europe, fundamentally different ecosystems and a global sea level more than 100 metres below its present position.
The comparison should be treated carefully. The transition out of the last glacial period unfolded over thousands of years, whereas current warming is occurring much more rapidly. Nor does a simple temperature comparison capture every difference between the two periods.
It nevertheless demonstrates an important principle: a change that appears modest when expressed as a global average can reshape the physical conditions of the planet.
This is also why global warming cannot be evaluated by looking only at the temperature in one city. Urban records are useful, but they may include local influences such as the urban heat-island effect. A growing city can become warmer because roads, buildings and reduced vegetation alter how heat is absorbed and released.
Climate datasets address this problem by combining urban and rural stations, identifying discontinuities, comparing nearby records and testing surface measurements against independent oceanic and satellite observations. The global trend remains present when urban stations are excluded or treated separately. It is not an artefact produced solely by the growth of cities.
The Earth’s energy imbalance
The physical basis of global warming can be understood through the Earth’s energy balance.
The planet receives energy from the Sun and releases energy back into space. Over sufficiently long periods, climate stability depends on a broad balance between the energy entering and leaving the system.
Greenhouse gases absorb and re-emit part of the infrared radiation that the Earth would otherwise lose to space. This natural greenhouse effect makes the planet habitable. Without it, average surface conditions would be far colder.
The scientific concern is not the existence of the greenhouse effect, but its intensification. Human activities have increased atmospheric concentrations of carbon dioxide, methane and other greenhouse gases. This produces additional radiative forcing: a change in the balance between incoming and outgoing energy.
When more energy enters the climate system than escapes, the planet accumulates heat. Surface warming is one expression of that imbalance, but not the only one. Energy is also absorbed by the oceans, used to melt ice and distributed through changes in the atmosphere, water cycle and land surface.
This broader energy perspective is important because short-term atmospheric temperature fluctuations can sometimes conceal the continuity of the underlying process. A year in which surface warming slows does not necessarily mean that the Earth has stopped accumulating energy.
The oceans as the principal heat reservoir
Most of the additional heat associated with the Earth’s energy imbalance does not remain in the air.
Current scientific assessments indicate that the oceans have absorbed approximately 90% of the excess heat accumulated in the climate system. The exact proportion varies slightly according to the period and methodology used, but the central conclusion is robust: the ocean is by far the largest reservoir of additional heat.
This matters for several reasons.
Water can store far more heat than air. Ocean temperatures also respond more slowly than atmospheric temperatures to short-term variability. As a result, ocean heat content provides one of the clearest measures of the planet’s longer-term energy accumulation.
Modern observations include measurements from research vessels, fixed instruments and the international Argo programme, whose autonomous floats descend through the ocean and return to the surface while recording temperature and salinity. Together, these systems show a sustained increase in ocean heat content over recent decades.
The warming is neither identical at every depth nor evenly distributed across every ocean basin. Currents, winds and natural variability move heat through the system. Some regions warm more rapidly than others, and changes in observational coverage create margins of uncertainty, particularly in deeper waters and earlier historical periods.
The overall direction, however, is difficult to reconcile with the idea that global warming is merely a statistical feature of land-based thermometers. The atmosphere and ocean are physically connected, yet they are measured through substantially different systems. Both indicate that the Earth is retaining more energy.
More than a line on a temperature graph
The evidence for global warming is sometimes presented as though it consisted of a single rising line. In reality, the conclusion is supported by a network of related observations.
The lower atmosphere has warmed.
The oceans have accumulated heat.
Many glaciers have lost mass.
Global mean sea level has risen.
The seasonal extent and age of Arctic sea ice have declined.
Atmospheric concentrations of greenhouse gases have increased substantially since industrialisation.
Not every indicator changes at the same rate, and no individual record is free from uncertainty. Some glaciers temporarily advance. Some regions experience periods of slower warming. Sea ice varies considerably from one year to the next. Climate models differ in their projections of particular regional effects.
Such variation is not evidence against global warming. It is what should be expected in a complex system influenced simultaneously by long-term forcing and natural variability.
The significant point is that different parts of the Earth system are responding in physically consistent ways. Warmer oceans expand. Land ice loss contributes water to the sea. Higher greenhouse gas concentrations reduce the rate at which energy escapes to space. The observed changes are not merely correlated; they are connected by well-established physical processes.
Climate science therefore does not ask readers to trust one graph, one laboratory or one international organisation. Its case rests on the agreement between multiple records whose uncertainties, although real, are not large enough to erase the overall pattern.
The planet has warmed. The oceans have stored most of the additional heat. Recent years have reached temperatures previously unseen in the instrumental record. These findings do not resolve every question about future impacts or appropriate public policy. They establish the physical foundation upon which those discussions must take place.
This is the first instalment of a three-part series examining what modern climate science says about global warming. Part two will explore the evidence preserved in glaciers, polar ice and sea-level records, and explain how atmospheric chemistry allows researchers to distinguish human influence from natural climate variability.
Further information & sources
Intergovernmental Panel on Climate Change (IPCC)
World Meteorological Organization (WMO)
National Oceanic and Atmospheric Administration (NOAA)
NASA climate science
Copernicus Climate Change Service
