Evidence That Climate Is Changing

🔍 Evidence That Climate Is Changing

One of the most important things to understand about climate change is that scientists do not rely on a single line of evidence. If only temperature records existed, there would be room for doubt — instruments might be poorly placed, records might be incomplete. But climate scientists have assembled multiple independent lines of evidence, each measuring something completely different, and they all tell the same story. This convergence of evidence across different fields and methods is why the scientific consensus on climate change is so strong.

1. Temperature Records

The global average surface temperature has risen approximately 1.2°C above pre-industrial levels (the 1850–1900 baseline) by 2023. The ten hottest years on record have all occurred since 2010. The year 2023 was the hottest single year ever recorded at 1.45°C above the pre-industrial baseline. These measurements come from thousands of weather stations on land and thousands of ocean buoys, independently operated by different national agencies — making coordinated error or fraud essentially impossible.

2. Ice Cores

Ice cores drilled from the Antarctic and Greenland ice sheets provide the most direct window into Earth's ancient climate. Layers of ice form each year like tree rings, trapping tiny air bubbles containing samples of the atmosphere at the time. Scientists can drill down and extract air from 800,000 years ago. These records show that CO₂ concentrations have fluctuated between roughly 180 ppm (ice ages) and 280 ppm (warm interglacials) over the past 800,000 years — and that temperature closely tracks CO₂ throughout. Since industrialisation, CO₂ has risen to 421 ppm in 2023 — a level not seen for at least 3 million years, far outside the natural range of variation. The rate of increase is also unprecedented: CO₂ is rising about 100 times faster than at the end of the last ice age.

3. Sea Level Rise

Global sea levels have risen by approximately 21–24 cm since 1880. The rate of rise has accelerated markedly: satellite data since 1993 shows a current rate of 3.7 mm per year, roughly double the average rate of the 20th century. Sea level rises for two interconnected reasons: thermal expansion (warmer water physically expands and takes up more volume) and melting ice (glaciers, the Greenland ice sheet, and parts of the Antarctic ice sheet are adding water to the ocean). The Greenland ice sheet alone contains enough water to raise global sea levels by approximately 7 metres if it melted entirely.

4. Glacier Retreat

Glaciers worldwide are retreating at accelerating rates. The Rhône Glacier in Switzerland lost 1.4 km in length in just 20 years (2000–2020) and has retreated more than 3 km since records began. Arctic sea ice extent has declined by roughly 13% per decade since satellite monitoring began in 1979; Arctic summer sea ice has lost approximately 40% of its area. The Himalayan glaciers — which supply freshwater to over a billion people across South Asia — are losing mass faster than almost anywhere else on Earth. The IPCC projects that two-thirds of Himalayan glacier volume could be lost by 2100 under high-emissions scenarios.

5. Shifting Seasons and Phenological Changes

Phenology — the study of seasonal biological events such as flowering, migration and hibernation — provides a biological record of warming. In the United Kingdom, spring is arriving approximately 26 days earlier than in the 1980s, based on records of first flowering dates kept by amateur recorders over centuries. Migratory bird species are arriving earlier and departing later. In the ocean, species distributions are shifting poleward as water temperatures rise.

6. Coral Bleaching

Coral reefs are among the most temperature-sensitive ecosystems on Earth. When water temperatures rise just 1–2°C above the normal summer maximum, corals expel the symbiotic algae (zooxanthellae) that give them colour and provide up to 90% of their energy — a process called bleaching. If temperatures remain elevated, corals starve and die. The Great Barrier Reef experienced four mass bleaching events between 2016 and 2024, with over 50% of the reef bleached in 2022 — a frequency that gives individual reefs no time to recover between events.