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examBoard: Cambridge
examType: IGCSE
lessonTitle: Evidence of Climate Change
Climate change is one of the biggest challenges facing our planet today. But how do we know it's actually happening? Scientists have collected many different types of evidence that all point to the same conclusion: our climate is changing and it's happening faster than at any time in Earth's recent history.
Key Definitions:
One of the clearest signs of climate change is the rise in global temperatures over time.
The Earth's average surface temperature has risen about 1.1°C since the late 19th century. Most of this warming has occurred in the past 40 years, with the seven most recent years being the warmest. The years 2016 and 2020 are tied for the warmest year on record.
Scientists use thermometer readings from weather stations on land, ships and buoys at sea to track global temperatures. For times before modern thermometers, they use "proxy" measurements from ice cores, tree rings and coral reefs.
The world's ice sheets and glaciers provide some of the most dramatic evidence of climate change.
Scientists drill deep into ice sheets in places like Antarctica and Greenland to extract ice cores. These cores contain trapped air bubbles from hundreds of thousands of years ago. By analyzing these bubbles, scientists can determine what the atmosphere was like in the past and how temperatures have changed.
Glaciers around the world are retreating at unprecedented rates. The Arctic sea ice is declining by about 13% per decade. Mountain glaciers in the Alps, Himalayas, Andes and Alaska are all shrinking. This melting is visible evidence of warming temperatures.
The Greenland Ice Sheet has been losing about 286 billion tonnes of ice per year between 1993 and 2016. Scientists use satellite measurements to track these changes. If the entire Greenland Ice Sheet were to melt, global sea level would rise about 7 metres, threatening coastal cities worldwide.
As water warms, it expands. Combined with melting ice, this is causing sea levels to rise globally.
Global sea level rose about 20 centimetres in the last century. The rate in the last two decades, however, is nearly double that of the last century and is accelerating slightly every year. This rise is partly due to melting ice and partly due to the expansion of seawater as it warms.
Many coastal areas are experiencing increased flooding, especially during high tides and storms. Places like Miami, Florida now experience "sunny day flooding" even without rain.
Low-lying island nations like the Maldives, Kiribati and Tuvalu are at risk of becoming uninhabitable as sea levels rise, forcing people to relocate.
Rising seas are accelerating coastal erosion, causing beaches to disappear and threatening homes and infrastructure built near shorelines.
Plants and animals are responding to climate change in ways we can observe.
Spring events like leaf growth and flowering are happening earlier. In the UK, spring now arrives about 2.5 days earlier per decade. Plants and trees are also shifting their ranges, with some species moving further north or to higher elevations as temperatures warm.
Many birds are migrating earlier in spring and later in autumn. Some animals are changing their ranges, moving to cooler areas. For example, in the UK, butterflies have been moving northward by about 10-25 km per decade.
Natural "record-keepers" help scientists understand climate patterns from before modern measurements began.
Trees add a new growth ring each year. In good growing years with plenty of warmth and moisture, rings are wider. In poor years, they're narrower. By studying these patterns in very old trees, scientists can reconstruct climate conditions from hundreds or even thousands of years ago.
This is the scientific name for studying tree rings. Some trees, like bristlecone pines, can live for thousands of years, providing very long climate records. Scientists have created climate timelines going back over 11,000 years using tree ring data.
Corals build their skeletons in layers, similar to tree rings. The chemistry of these layers changes with water temperature and other conditions, allowing scientists to reconstruct past ocean conditions. Unfortunately, warming oceans are now causing widespread coral bleaching.
While no single weather event can be attributed solely to climate change, the pattern of extreme weather is changing in ways consistent with what scientists expect in a warming world.
Heatwaves are becoming more frequent and intense. The 2019 European heatwave saw temperature records broken in many countries, with the UK recording its highest-ever temperature of 38.7°C in Cambridge.
Warmer oceans provide more energy for storms. While the total number of hurricanes/typhoons may not increase, their intensity is. Category 4 and 5 storms have become more common since the 1980s.
Climate change is making wet areas wetter and dry areas drier. Heavy downpours are becoming more common, leading to increased flooding in many regions.
The 2019-2020 Australian bushfire season, known as the "Black Summer," was exceptionally severe. About 18.6 million hectares burned, destroying over 5,900 buildings and killing at least 34 people. While bushfires are natural in Australia, climate change created conditions that made these fires more extreme. Record-breaking heat and drought dried out vegetation, creating perfect conditions for fires to spread rapidly and burn intensely.
The oceans have absorbed about 30% of the carbon dioxide released by human activities, changing their chemistry.
When CO₂ dissolves in seawater, it forms carbonic acid, making the ocean more acidic. Since the beginning of the Industrial Revolution, the pH of surface ocean waters has fallen by 0.1, representing about a 30% increase in acidity. This might not sound like much, but even small changes in pH can have big effects on marine life.
More acidic water makes it harder for shellfish, corals and some plankton to build their shells and skeletons. These creatures are vital parts of ocean food webs, so changes affect the entire marine ecosystem.
Scientists measure ocean pH directly using sensors on research vessels, buoys and autonomous underwater vehicles. They can also determine past ocean acidity by studying the shells of marine organisms preserved in ocean sediments.
The evidence for climate change doesn't rely on just one measurement or one type of data. Instead, multiple independent lines of evidence all point to the same conclusion: our planet is warming and human activities are the main cause.
This "fingerprint" of human-caused climate change includes:
Scientists from around the world review this evidence through the Intergovernmental Panel on Climate Change (IPCC). Their latest report states that it is "unequivocal" that human influence has warmed the atmosphere, ocean and land.
Over 97% of actively publishing climate scientists agree that human activities are causing climate change. This level of agreement is similar to the scientific consensus that smoking causes cancer. The evidence has become so overwhelming that the debate among scientists is no longer about whether climate change is happening, but about how quickly it's happening and what the precise impacts will be.
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