What the evidence says about global warming — part two
- Editorial Team SDG13
- Jul 21
- 6 min read
Published on 21 July 2026 at 03:59 GMT
By Editorial Team SDG13
One of the defining characteristics of robust scientific knowledge is that it rarely depends on a single line of evidence. A hypothesis becomes progressively stronger when independent observations, gathered using different methods and interpreted by different research teams, point towards the same conclusion.
Climate science offers an unusually clear example of this principle.
The warming measured by thermometers is only one part of a much broader body of evidence. Glaciers, polar ice sheets, sea level, atmospheric chemistry and even the vertical structure of the atmosphere provide independent records of how the Earth’s climate system is changing. None of these observations relies upon the others, yet they tell a remarkably consistent story.
This convergence is one of the principal reasons why the scientific confidence surrounding global warming has strengthened over recent decades.
Glaciers as long-term climate indicators
Glaciers respond slowly to changes in climate. Unlike daily weather, which may fluctuate dramatically over short periods, glaciers integrate changes in temperature and snowfall over many years.
For this reason, they are often described as natural archives of climate.
According to the World Glacier Monitoring Service (WGMS), the vast majority of monitored glaciers around the world have experienced a sustained loss of mass over recent decades. The cumulative losses observed since the late twentieth century are greater than any previously documented during the instrumental period.
It is important, however, to understand what these observations do and do not imply.
Not every glacier is retreating every year.
Some glaciers have remained relatively stable over limited periods. Others have temporarily advanced because increased snowfall has outweighed summer melting. Local geography, altitude, prevailing winds and precipitation patterns all influence glacier behaviour.
Climate scientists therefore avoid drawing conclusions from individual glaciers.
Instead, they examine thousands of glaciers distributed across different continents and climatic regions. When considered collectively, the evidence indicates a widespread reduction in glacier mass consistent with a warming climate.
This distinction illustrates an essential feature of scientific reasoning.
Individual exceptions do not necessarily invalidate an overall trend. On the contrary, understanding why exceptions occur often improves our understanding of the broader system.
Greenland and Antarctica: similar questions, different answers
Public discussion frequently treats the polar ice sheets as though they behaved in the same way.
In reality, Greenland and Antarctica are very different systems.
The Greenland Ice Sheet occupies a comparatively warmer environment and experiences substantial summer melting across parts of its surface. Satellite gravimetry, satellite altimetry and field observations all indicate that Greenland has lost a significant amount of ice during recent decades, although the precise annual losses vary from year to year.
Antarctica presents a more complicated picture.
East Antarctica, which contains the majority of the continent’s ice, has remained comparatively stable in many areas and has occasionally experienced temporary increases in snowfall. By contrast, parts of West Antarctica and several Antarctic ice shelves have shown substantial ice losses associated with interactions between warmer ocean waters and floating ice shelves.
These regional differences sometimes create confusion in public debate.
Reports highlighting temporary snowfall gains in one part of Antarctica are occasionally presented as evidence against global warming. Yet this interpretation overlooks the fact that Antarctica is an immense continent with highly contrasting climatic conditions.
Scientific assessments therefore evaluate the continent region by region rather than treating it as a single uniform system.
This more nuanced approach does not weaken the evidence.
It strengthens it.
Science progresses by describing complexity rather than simplifying it away.
The Arctic: one of the clearest observational records
If Antarctica illustrates complexity, the Arctic provides one of the clearest observational records available.
Since 1979, satellite observations have monitored Arctic sea ice continuously, producing one of the longest and most consistent datasets in climate science.
According to the National Snow and Ice Data Center (NSIDC), the minimum summer extent of Arctic sea ice has declined by approximately 12% per decade relative to the 1981–2010 average.
Surface extent, however, tells only part of the story.
Researchers have also documented substantial reductions in the thickness and age of Arctic sea ice.
Historically, much of the Arctic Ocean was covered by thick multi-year ice capable of surviving several summers. Today, a larger proportion consists of thinner seasonal ice that forms during winter and melts more readily during summer.
The Arctic therefore illustrates two related changes occurring simultaneously: less ice and younger ice.
Neither observation proves, by itself, that human activity is responsible.
Together with the broader climate record, however, they form part of a consistent physical picture.
Sea level: another independent measurement
Few climate indicators have been monitored using such different technologies as sea level.
For more than a century, tide gauges installed in ports around the world have recorded changes in local sea level.
Since the early 1990s, satellite altimeters have provided global measurements of the height of the ocean surface with centimetre-scale precision.
These independent systems broadly agree.
Current scientific assessments indicate that global mean sea level has risen by approximately 20 centimetres since the beginning of the twentieth century, although the exact estimate depends upon the reconstruction method and reference period used.
The increase is also not occurring at a constant rate.
Modern observations suggest that the rate of sea-level rise has accelerated during recent decades.
Two principal mechanisms explain this change.
The first is thermal expansion.
As seawater absorbs heat, it expands. Even without adding extra water, a warmer ocean occupies slightly more volume.
The second mechanism involves the addition of water from melting glaciers and continental ice sheets.
Together, these processes account for most of the observed increase.
It should also be remembered that sea level does not rise uniformly everywhere.
Ocean currents, vertical land movement, gravitational effects associated with changing ice sheets and regional wind patterns all influence local sea level.
Consequently, coastal communities experience different rates of change despite sharing the same global ocean.
How do scientists know the warming is not simply natural?
This question lies at the centre of much public discussion.
The Earth’s climate has always changed.
Ice ages have come and gone. Volcanic eruptions have temporarily cooled the atmosphere. Variations in the Earth’s orbit have altered the distribution of sunlight over thousands of years. Solar activity itself changes over time.
Climate scientists fully acknowledge these natural influences.
The question is not whether natural climate variability exists.
It is whether natural factors alone can explain the warming observed since the mid-twentieth century.
The evidence suggests they cannot.
One of the strongest indicators comes from atmospheric carbon dioxide.
Ice cores extracted from Antarctica preserve ancient air bubbles, allowing scientists to reconstruct atmospheric composition over hundreds of thousands of years.
Before the Industrial Revolution, atmospheric carbon dioxide concentrations remained close to approximately 280 parts per million.
Today they are around 425 parts per million, and in 2025 the seasonal peak measured at Mauna Loa exceeded 430 parts per million for the first time.
This increase alone, however, does not identify its origin.
The decisive evidence comes from carbon isotopes.
Carbon exists in several naturally occurring forms, or isotopes. Fossil fuels contain a characteristic isotopic signature because they originate from ancient organic matter that became buried millions of years ago.
As fossil fuels are burned, this distinctive signature becomes detectable in atmospheric carbon dioxide.
Measurements made over many decades show precisely this pattern.
In other words, the additional carbon accumulating in the atmosphere is not merely increasing.
Its chemical fingerprint identifies fossil fuel combustion as the dominant source.
The atmosphere itself provides another clue
Another important line of evidence comes from the vertical structure of the atmosphere.
If the recent warming were caused primarily by increased solar output, scientists would expect both the lower and upper atmosphere to warm together.
Observations show something different.
The troposphere, the lowest layer where weather occurs, has warmed.
The stratosphere, above it, has generally cooled.
This combination is precisely what atmospheric physics predicts when greenhouse gases reduce the rate at which infrared radiation escapes into space.
For climate scientists, this pattern represents one of the most convincing “fingerprints” of an enhanced greenhouse effect.
No single observation establishes the conclusion on its own.
Temperature records, atmospheric chemistry, glacier behaviour, sea-level rise, ocean heat content and the vertical structure of the atmosphere each contribute independent pieces of evidence.
Together, they describe a coherent physical system responding in ways that are mutually consistent.
Science progresses by reducing uncertainty
Perhaps the greatest misunderstanding surrounding climate science concerns uncertainty itself.
Uncertainty is often portrayed as evidence that scientists do not understand the climate.
In reality, uncertainty is an essential feature of scientific research.
Researchers continue to investigate cloud feedbacks, ice-sheet dynamics, regional precipitation, ecosystem responses and many other questions whose precise outcomes remain difficult to quantify.
These uncertainties matter because they influence projections of future change.
They do not, however, eliminate the extensive observational evidence describing changes that have already occurred.
Science rarely deals in absolute certainty.
Instead, it evaluates probabilities, tests competing explanations and refines conclusions as new evidence becomes available.
Climate science is no exception.
If anything, it has become progressively stronger because successive observations have reinforced rather than weakened the central conclusions reached over previous decades.
This is the second instalment of a three-part series examining what modern climate science says about global warming. The final article will explore where genuine scientific uncertainty still exists, examine the work of organisations such as the IPCC and Project Drawdown, and consider why distinguishing scientific evidence from political debate remains essential.
Further information
World Glacier Monitoring Service (WGMS)
National Snow and Ice Data Center (NSIDC)
Intergovernmental Panel on Climate Change (IPCC)
World Meteorological Organization (WMO)
NASA Earth Observatory
NOAA
