Global warming and climate change

The resource shows that both the global temperature anomaly and the concentration of atmospheric CO₂ increase over the period from 1850 to the present.

CO₂ rises from roughly 285 ppm in 1850 to over 410 ppm today, while the temperature anomaly rises by about 1 °C. Both increases are steeper after the mid-20th century, so the rate of warming accelerates.

Importantly, the temperature and CO₂ trends rise together, which provides evidence that the warming is linked to rising greenhouse-gas concentrations.

Proxy evidence is natural evidence used to reconstruct climate before thermometer records began (around 1850).

Ice cores contain trapped air bubbles that record past concentrations of CO₂ and methane, while isotopes in the ice indicate past temperatures, going back hundreds of thousands of years.

Tree rings (dendrochronology) and fossils provide further proxies: wider tree rings indicate warmer, wetter growing years, and the species found as fossils (such as pollen) indicate the climate in which they lived.

The greenhouse effect operates within the global energy budget — the balance between incoming shortwave solar radiation and outgoing longwave radiation.

Incoming shortwave radiation from the Sun passes largely through the atmosphere and warms the Earth's surface. The surface then re-emits this energy as longwave (infrared) radiation. Greenhouse gases such as carbon dioxide and methane absorb some of this longwave radiation and re-emit part of it back towards the surface, a process called back-radiation. This is the natural greenhouse effect, which keeps the planet habitable.

The enhanced greenhouse effect occurs when human activity adds extra greenhouse gases to the atmosphere — for example, CO₂ from burning fossil fuels and methane from agriculture. With more greenhouse gases present, more outgoing longwave radiation is absorbed and re-emitted downwards, and less escapes to space.

This means the energy budget tips towards a net gain of energy, known as positive radiative forcing. To restore balance, the surface warms, so global temperatures rise. The effect is amplified by positive feedbacks such as the ice–albedo feedback, where melting ice exposes darker surfaces that absorb even more radiation.

Human activities affect the energy budget in three main ways, all increasing the net energy gained by the Earth.

First, human activities emit greenhouse gases. Burning fossil fuels for energy, industry and transport releases large amounts of CO₂, agriculture and landfill release methane, and fertilisers release nitrous oxide. More greenhouse gases trap more outgoing longwave radiation, strengthening the enhanced greenhouse effect.

Second, human activities change the absorption and storage of greenhouse gases. Deforestation removes trees that absorb CO₂ through photosynthesis and releases the carbon they stored, while soil disturbance and warming oceans weaken natural carbon sinks. This leaves more CO₂ in the atmosphere.

Third, human activities change albedo, the reflectivity of the surface. As warming melts ice and snow, darker ocean and land are exposed, which absorb more solar radiation and cause further warming — a positive feedback.

Together, more greenhouse gases, weaker carbon sinks and lower albedo push the energy budget towards a net gain of energy, raising global temperatures.

Recent global warming could in principle result from human (anthropogenic) factors acting through the enhanced greenhouse effect, or from physical factors such as solar output, volcanic activity and the El Niño Southern Oscillation (ENSO). This essay assesses whether human activity is the main cause, and argues that while physical factors are real and important for short-term variability, the rapid, sustained warming since the mid-20th century is best explained by human activity.

There is a strong case that human activity is the main cause. Burning fossil fuels, agriculture and industry have added large quantities of greenhouse gases — CO₂, methane, nitrous oxide and F-gases — to the atmosphere, strengthening the enhanced greenhouse effect so that more outgoing longwave radiation is trapped. Deforestation and soil disturbance have weakened the carbon sinks that would otherwise remove CO₂, and the loss of ice has lowered albedo, exposing darker, more absorbent surfaces. The evidence supports this attribution: instrumental temperature records since 1850 rise closely with measured CO₂ (the Keeling Curve), and ice cores show that recent CO₂ concentrations are far above the natural range of the last several hundred thousand years. The close correspondence between rising CO₂ and rising temperature is difficult to explain by natural causes alone.

However, physical factors must be weighed fairly, because climate has always changed naturally. Variations in solar output alter incoming shortwave radiation over the long term; large volcanic eruptions inject aerosols that reflect sunlight and cause short-term cooling; and ENSO redistributes ocean heat, making individual years warmer (El Niño) or cooler (La Niña). These factors genuinely affect the energy budget. The crucial point, though, is their timescale and direction: measured solar output has not risen enough in recent decades to explain the warming, volcanic eruptions cause cooling rather than warming, and ENSO produces variability around the trend rather than the trend itself.

The two sets of factors are best seen as interacting. Physical factors largely explain the year-to-year and decade-to-decade wobbles — a strong El Niño can push a year to record warmth, and a major eruption can mask warming for a year or two — while the underlying, sustained rise tracks human greenhouse-gas emissions. In other words, physical factors explain the short-term variability and human factors explain the long-term trend.

It is also important to acknowledge that atmospheric processes are complex and have many interdependencies. Positive feedbacks such as the ice–albedo and water-vapour feedbacks amplify warming, while uncertainties (for example in cloud behaviour) make the precise magnitude of future change harder to predict. This complexity means judgements should be measured rather than absolute.

Overall, I largely agree with the view. Human activity, acting through the enhanced greenhouse effect, is the main cause of recent global warming, because the evidence links the sustained post-1950 rise to rising greenhouse-gas concentrations, and the physical factors are either too small, in the wrong direction, or only short-term in their effect. The fairest judgement is that physical factors remain important for natural variability, but human activity is the dominant driver of the recent warming trend.

Whether the evidence for global warming is convincing depends on the range, consistency and reliability of the different types of evidence available. This essay assesses that evidence and argues that, taken together, it is highly convincing, while recognising that individual proxies have limitations and that attributing warming to human causes is a separate, slightly more complex question.

The direct, instrumental evidence is strong. Reliable global temperature records since about 1850 show a clear rise in average surface temperature, with a marked acceleration after the mid-20th century. These are direct thermometer measurements, not inferences, which makes them convincing. They are reinforced by rising ocean temperatures, since the oceans absorb most of the extra heat.

The physical, observable evidence adds further weight. Sea levels are rising through the thermal expansion of warming water and meltwater from glaciers and ice sheets; Arctic sea-ice extent is shrinking, especially in summer; and glaciers and the Greenland and Antarctic ice sheets are losing mass. Because these independent lines of evidence — temperature, sea level, sea ice and ice mass — all point the same way, the overall picture is highly consistent and therefore convincing.

Proxy evidence extends the record back beyond thermometers and is particularly powerful. Ice cores contain trapped air bubbles that directly record past CO₂ and methane, alongside isotopes that indicate past temperature, while tree rings and fossils provide further reconstructions. These show that recent CO₂ levels and warming are unusual against the natural variation of the past several hundred thousand years.

There are limitations that a balanced answer should acknowledge. Pre-1850 temperatures are inferred rather than measured, and proxies such as tree rings can be affected by factors other than temperature (for example moisture or soil). Early instrumental records have gaps and were not globally even. The climate system is also complex, with natural variability from ENSO and volcanic eruptions adding short-term noise. However, these limitations weaken individual proxies, not the combined picture: where many independent methods agree, the conclusion is robust.

Overall, I strongly agree that the evidence the world is warming is convincing. The combination of direct temperature records, multiple physical indicators (sea level, sea ice, ice mass) and long proxy records all point consistently to warming, and the agreement between independent lines of evidence is precisely what makes the case persuasive. The remaining uncertainties concern the precise magnitude and, separately, the attribution of warming to human causes, rather than whether warming is occurring at all.