Evidence for recent climate change and its links to human activity

Overview

Happening to sea ice, mountain glaciers and the ice sheets, happening in the ocean besides the sea surface warming and assessed scientific conclusion about human influence on the climate.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does the instrumental temperature record show?

   Model answer

   Global mean surface temperature, compiled from land weather stations and ship and buoy measurements of sea surface temperature since about 1850, has risen by a little over one degree Celsius, with the decade 2011 to 2020 assessed at about 1.1 degrees above the 1850 to 1900 baseline and subsequent years higher still. The rise is not steady: there was warming to about 1940, a plateau to the mid-1970s attributed largely to industrial sulphate aerosol pollution reflecting sunlight, and then rapid and continuing warming since. Warming is greater over land than sea and far greater in the Arctic. Every one of the warmest years in the record has occurred since 2015, and several independent research groups compiling the data separately obtain the same result.

2. 2

   What has happened to sea level, how is it measured, and what are the two causes?

   Model answer

   Global mean sea level has risen by roughly 20 centimetres since 1900, and the rate has accelerated from about 1.4 millimetres a year early in the twentieth century to about 3.7 millimetres a year in recent decades. It is measured by a long network of coastal tide gauges, which must be corrected for land that is itself rising or sinking, and since 1993 by satellite radar altimetry, which measures the ocean surface globally and directly. There are two causes, both consequences of warming. Thermal expansion: water expands as it warms, and most of the extra heat has gone into the ocean. And the addition of water from melting mountain glaciers and from the Greenland and Antarctic ice sheets. Both contribute substantially, with the ice contribution growing.

3. 3

   What is happening to sea ice, mountain glaciers and the ice sheets?

   Model answer

   Arctic sea ice has declined sharply, with the September minimum extent falling by roughly 13 per cent per decade since satellite monitoring began in 1979, and the ice that remains is thinner and younger. Mountain glaciers are in retreat almost everywhere they are monitored, from the Alps and Andes to the Himalaya and Rockies, with negative mass balance recorded in nearly every year of recent decades and many small glaciers already gone. Greenland and Antarctica are both losing mass overall, measured by satellite gravimetry, by altimetry and by comparing snowfall input with outflow. Northern hemisphere spring snow cover has decreased and permafrost temperatures have risen. Antarctic sea ice behaved differently for some years, which is a genuine complication, but it too has recently fallen to record lows.

4. 4

   What is happening in the ocean besides the sea surface warming?

   Model answer

   Two things of great importance. First, ocean heat content is rising through the whole depth measured, and since the ocean has vastly greater heat capacity than the atmosphere it has absorbed over 90 per cent of the extra energy trapped in the system; this measurement is the most reliable single indicator that the planet is gaining energy, because it is much less affected by year-to-year variability than surface air temperature. Second, the ocean has absorbed roughly a quarter to a third of human carbon dioxide emissions, which reacts with seawater to form carbonic acid, so surface pH has fallen by about 0.1 units, and because the scale is logarithmic that is a rise of roughly 25 to 30 per cent in hydrogen ion concentration. This acidification makes it harder for corals, molluscs and some plankton to build calcium carbonate shells and skeletons.

5. 5

   What biological and seasonal evidence is there for recent climate change?

   Model answer

   Phenological records, many of them kept by amateur naturalists for a century or more, show spring events happening earlier: earlier leaf burst and blossom, earlier arrival of migrant birds, earlier frog spawning and earlier egg laying. Growing seasons in mid and high latitudes have lengthened by a couple of weeks. Species ranges are shifting polewards and to higher altitudes on mountains, both on land and in the sea, where commercial fish stocks have moved north. Tree lines and the limits of permafrost-dependent vegetation are moving. Repeated mass coral bleaching events, unknown before the 1980s, have become frequent. This evidence is valuable because it is independent of thermometers and comes from thousands of separate records worldwide.

6. 6

   What has happened to weather extremes?

   Model answer

   Heatwaves have become more frequent, more intense and longer, and cold extremes less frequent, which is exactly what a shift in the whole temperature distribution produces. Heavy rainfall events have intensified in most land regions, because warmer air holds around 7 per cent more moisture for each degree of warming, so when it does rain more water is available to fall. Drought has worsened in some regions, particularly the Mediterranean, south-west North America and southern Africa, through a combination of reduced rainfall and increased evaporation. Fire weather seasons have lengthened. Tropical cyclones show a smaller and less certain signal in frequency but evidence of a higher proportion reaching the most intense categories, higher rainfall rates and higher storm surge because of sea-level rise.

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