Climate change: How do we know it is happening and caused by humans?

Climate change: How can we be sure it is occurring and has been caused by humans? Published 2 hours ago
Image source, Frans Lemmens

Scientists and politicians agree that climate change is causing a global crisis.

What evidence is there for global warming? And how can we prove it's caused by humans?

How can we tell if the world is warming?

Since the Industrial Revolution, our planet has been rapidly warming.

Since 1850, the average surface temperature has increased by 1.1C. Additionally, the temperature has been higher in each decade since 1850 than it was during the middle of 19th century.

These conclusions are based on millions of measurements taken in different locations around the globe. Temperature readings are collected from weather stations located on land, ships, and satellites.

Multiple scientific teams have come up with the same conclusion - an increase in temperature that coincides with the industrial revolution.

Image source: Reuters Image caption. Turkey was among the worst-hit countries by wildfires this summer

Scientists can also reconstruct temperature fluctuations back in time.

The past climate signature is evident in tree rings, ice cores and lake sediments.

This gives context to the current stage of global warming. Scientists estimate that the Earth hasn’t been this warm in 125,000 years.

How can we prove that humans are responsible worldwide warming?

The key link between human activities and temperature rise is made by greenhouse gases, which trap the Sun's warmth. Because of its abundance in our atmosphere, carbon dioxide (CO2) is the most important.

It can also be determined that the Sun's heat is being trapped by CO2. Satellites reflect less heat from Earth into space at the wavelengths where CO2 absorbs radiated energie.

This greenhouse gas is released when fossil fuels are burned and trees are cut down. Both of these activities were a major contributor to atmospheric CO2's rise in the 20th century.

Source: Getty Images

We can prove definitively the source of this additional CO2. Carbon from fossil fuels is unique in its chemical signature.

Both tree rings and polarice record changes in atmospheric chemicality. They show that carbon, specifically from fossil sources, has increased significantly in the past 1850.

Analyses show that atmospheric CO2 has remained at the same level for 800,000. However, since the Industrial Revolution, atmospheric CO2 has increased to nearly 420ppm.

Climate models are computer simulations that show what the temperature would look like if there were no massive greenhouse gas emissions from humans.

They show that there would have been very little global warming and perhaps some cooling over the 20th and 21st Centuries if natural factors hadn't been influencing the climate.

Models can only explain temperature increases when they are influenced by human factors.

Image source: Getty Images Image caption: The number of weather-related catastrophes has increased by five over the past 50 years

How do humans have an impact on the planet?

It is expected that the Earth's current heat will cause major changes in the world around it.

These changes are consistent with what scientists expect to see in the face of human-induced global warming. These include:

Rapid melting of the Antarctic and Greenland ice sheets

Over 50 years, the number of weather-related catastrophes has increased by five.

The global sea level rose by 20cm (8ins), over the past century, and is still rising

The oceans have become 40% acider since the 1800s, which has adversely affected marine life

Wasn't it warmer in past years?

There have been many hot periods in the Earth's history.

For example, temperatures were so high back in 92 million years that there weren't polar ice caps. Crocodile-like creatures also lived as far north at the Canadian Arctic.

In the past, the sea level was 25m (80ft), higher than it is today. Most coastal cities around the globe would be submerged if there was a rise of between 5-8m and 26ft.

Evidence is overwhelming that mass extinctions of species occurred during these times. Climate models also suggest that the tropics may have been "dead zones", which could have meant that most species cannot survive in these areas.

These temperature fluctuations can be caused by many phenomena including the Earth's wobble as it orbits around the Sun for long periods of time, volcanic eruptions, and short-term climate cycles like El Nio.

Groups of climate "sceptics", who have been doubting the scientific basis for global warming for many years, have been arguing against it.

However, almost all scientists who regularly publish in peer-reviewed journals agree on the current causes for climate change.

In 2021, a UN report stated that it was "unambiguous" that human influence had warmed the oceans, land and atmosphere.

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