Scientists and politicians say we are facing a planetary crisis due to climate change.
But what is the evidence of global warming and how do we know it’s man-made?
How do we know the world is warming up?
Our planet has warmed rapidly since the dawn of the industrial revolution.
The the average temperature on the earth’s surface has increased by about 1.1 ° C since 1850. Furthermore, each of the past four decades has been warmer than any that preceded it, since the mid-19th century.
These conclusions come from the analysis of millions of measurements collected in different parts of the world. Temperature readings are collected from terrestrial, naval and satellite meteorological stations.
Multiple independent teams of scientists have achieved the same result: a spike in temperatures coinciding with the start of the industrial age.
Scientists can reconstruct temperature fluctuations even further back in time.
Tree rings, ice cores, lake sediments and corals all record a signature of the past climate.
This provides the much needed context for the current warm-up phase. In fact, scientists estimate that the Earth hasn’t been that hot for around 125,000 years.
How do we know that humans are responsible for global warming?
Greenhouse gases, which trap heat from the sun, are the crucial link between rising temperatures and human activities. The most important is carbon dioxide (CO2), due to its abundance in the atmosphere.
We can also say that it is CO2 that traps the Sun’s energy. Satellites show less heat from the Earth as it escapes into space at exactly the wavelengths at which CO2 absorbs radiated energy.
The burning of fossil fuels and the felling of trees lead to the release of this greenhouse gas. Both activities exploded after the 19th century, so it is not surprising that atmospheric CO2 increased over the same period.
There is a way to definitively show where this extra CO2 comes from. The carbon produced by burning fossil fuels has a distinctive chemical signature.
Tree rings and polar ice both record changes in atmospheric chemistry. When examined they show that carbon – particularly from fossil sources – has increased significantly since 1850.
Analysis shows that atmospheric CO2 did not exceed 300 parts per million (ppm) for 800,000 years. But since the industrial revolution, the CO2 concentration has risen to its current level of nearly 420 ppm.
Computer simulations, known as climate models, were used to show what would have happened to temperatures without the huge amounts of greenhouse gases released by humans.
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They reveal that there would have been little global warming – and possibly some cooling – over the course of the 20th and 21st centuries, if only natural factors had affected the climate.
Only when human factors are introduced can models explain the rise in temperature.
What impact do humans have on the planet?
The level of warming the Earth has already experienced is expected to cause significant changes in the world around us.
Real-world observations of these changes match the patterns scientists expect to see with human-induced warming. They include:
- The Greenland and Antarctic ice sheets are melting rapidly
- The number of climate-related disasters has increased fivefold in 50 years
- Global sea level has risen by 20 cm (8 inches) over the past century and is still rising
- Since the 1800s, the oceans have become about 40% more acidic, affecting marine life
But wasn’t it warmer in the past?
There have been several warm periods during Earth’s past.
About 92 million years ago, for example, temperatures were so high that there were no polar ice caps and crocodile-like creatures lived as far north as the Canadian Arctic.
This shouldn’t comfort anyone, however, because humans weren’t around. At times, in the past, the sea level was 25 m (80 ft) higher than it is today. A height of 5-8 m (16-26 feet) is considered sufficient to submerge most of the coastal cities of the world.
There is abundant evidence of mass extinctions of life during these times. And climate models suggest that, at times, the tropics could have become “dead zones”, too hot for most species to survive.
These fluctuations between heat and cold have been caused by a variety of phenomena, including the way the Earth oscillates as it orbits the Sun for long periods, volcanic eruptions, and short-term climate cycles such as El Niño.
For many years, groups of so-called climate “skeptics” have questioned the scientific basis of global warming.
However, virtually all scientists who regularly publish in peer-reviewed journals now agree on the current causes of climate change.
A key UN report released in 2021 stated that “it is unequivocal that human influence has warmed the atmosphere, the oceans and the earth.”
The COP26 world climate summit in Glasgow in November is considered crucial if climate change is to be kept under control. Nearly 200 countries are being asked for their plans to reduce emissions and this could lead to major changes in our daily lives.
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