Describe the evidence that shows the Earth's climate is changing

What this dot point is asking

SEAB wants you to describe the evidence that the Earth's climate is changing and, in particular, warming. The central insight is that the case rests on many independent lines of evidence pointing the same way: direct temperature records, the shrinking of ice, rising seas, and longer-term natural records called proxies. No single measurement proves it; the strength is in the agreement of them all.

The answer

Rising global temperatures

The most direct evidence is the instrumental temperature record from thermometers at weather stations, ships and buoys worldwide. It shows that global average temperature has risen by about $1\ \text{C}$ since around 1900, with most of the warming occurring since the 1970s and recent years among the warmest on record. The trend is upward and accelerating.

Shrinking ice and glaciers

A warming world melts ice, and the evidence is widespread:

• Mountain glaciers are retreating on almost every continent, visible in repeat photographs taken decades apart.
• Arctic sea ice has shrunk in extent and thickness, measured by satellites.
• Ice sheets in Greenland and Antarctica are losing mass, tracked by satellites.

Rising sea levels

Global sea level is rising, measured by tide gauges and satellites. There are two causes, both linked to warming: the melting of land ice (glaciers and ice sheets) adds water to the oceans, and the thermal expansion of seawater, as warmer water takes up more space, raises the level further.

Longer-term proxy evidence

Thermometers only go back about 150 years, so scientists use proxy records, natural archives of past climate:

• Ice cores from Greenland and Antarctica trap bubbles of ancient air, revealing past temperatures and greenhouse gas levels over hundreds of thousands of years.
• Tree rings are wider in warmer, wetter years and narrower in colder ones, giving a year-by-year record.
• Ocean sediments and pollen record past conditions over very long timescales.

These show that present greenhouse gas levels and the speed of warming are unusual compared with the natural past.

Examples in context

Example 1. Rising sea levels around Singapore. Tide-gauge and satellite records show sea levels rising in the waters around Singapore, in line with the global trend. As a low-lying island state, Singapore takes this evidence seriously, which is why agencies study local sea-level rise closely and plan coastal protection. The local measurements are one strand of the global evidence that the climate is warming.

Example 2. Retreating Himalayan glaciers. Glaciers in the Himalayas, which feed major Asian rivers, have been retreating over recent decades, documented by repeat photography and satellite monitoring. Their shrinkage is both clear evidence of regional warming and a warning, since hundreds of millions of people downstream depend on the meltwater. It shows how glacier evidence links directly to human concerns.

Try this

Q1. State two pieces of evidence, other than temperature records, that the climate is warming. [2 marks]

• Cue. Shrinking glaciers and sea ice (ice sheets losing mass), and rising global sea levels measured by tide gauges and satellites.

Q2. Explain why scientists use ice cores to study past climate. [2 marks]

• Cue. Ice cores trap bubbles of ancient air whose composition reveals past temperatures and greenhouse gas levels going back hundreds of thousands of years, extending the climate record far beyond the roughly 150 years of thermometer data.

Q3. Explain why rising sea levels are partly caused by warming even without ice melting. [2 marks]

• Cue. As seawater warms it expands and takes up more space (thermal expansion), so the volume of the oceans increases and the sea level rises even before any extra water is added from melting land ice.