climate of our environment is not a modern one. Indeed, there are accounts of ancient Greeks and 19th-century Americans debating how cutting down forests might bring more rainfall to a region, or less. Already in the 1930s, people realized that the region of the United States and the North Atlantic had warmed significantly during the previous half‑century. At the time, it was unclear why this had happened and the potential for this change to have occurred due to human activity was not considered a real possibility.
It was really only G. S. Callendar who insisted that greenhouse warming was the cause. The view did not gain any real traction until the 1950s, when scientists were able to employ improved techniques and calculations to confirm that carbon dioxide could build up in the atmosphere raising temperatures.
By 1960, the painstaking measurements of C. D. Keeling provided irrefutable proof that the levels of greenhouse gases were increasing, year by year.
Today, we know that the global average temperature has risen by about 1°C since the late 1900’s which among others has led to declining glaciers and sea ice while the global sea level has risen by about 20cm. At the same time, increased CO2 levels are causing the oceans to become more acidic impacting aquatic ecosystems.
Even though scientists do not have a clear threshold to identify when climate change moves from safe to dangerous, disruptions and irreversible losses of natural habitats and resources is going to happen even with a 2°C temperature rise 1.
This is particularly concerning if one considers that with not action to tackle climate change, global temperatures could increase by 7°C or more2. But how does climate change happen?
What causes global warming?
The scientific consensus is that human activity is causing climate change. More specifically, in its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC), made up of 1,300 independent scientific experts from countries across the world and operating under the auspices of the United Nations, concluded that the pro probability that human activities have warmed our planet over the past 50 years is over 95%3. This is as clear as science can be.
While in the past our climate did undergo radical changes, it did so over many thousands of years, because of a number of reasons; this is not the same kind of change as we are seeing today. For example, at times, the UK was hot enough for hippos lived in Norfolk while at other times ice has covered the country for tens of thousands of years4. Global climate change has usually occurred very slowly. Today our climate is changing very quickly.
What is happening, and what types of human activities have affected our climate?
Life on our planet depends on energy coming from the Sun. When the light from the Sun reaches the Earth’s atmosphere, it passes through the air and clouds to the surface, where it is absorbed and radiated upward in the form of infrared heat. 90% of this heat is then absorbed by greenhouse gases and radiated back towards the surface, which is then warmed to about 15 degrees Celsius (59 degrees Fahrenheit) which is a temperature that enables life on Earth. Many natural activities can alter the gases in our atmosphere, for example volcanoes. But over the last two centuries, changes in the atmosphere have come about as a result of the Industrial Revolution.
Studies have shown that industrial activities have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million.
This is because they have resulted in emissions of what we now call greenhouse gases such as carbon dioxide, methane and nitrous oxide5. These gases, when released into the atmosphere, block heat from escaping. They are therefore very important in helping maintain the optimum temperature for sustaining life on Earth.
However, if the concentration of those gases increases this can overheat the atmosphere. For example, once carbon is emitted into the atmosphere, it sticks around for hundreds of years, some of it even longer, trapping heat. The effects of this are cumulative, growing more severe with time. Today, so much carbon has been allowed to accumulate in the atmosphere that to keep warming below the internationally agreed-upon target of 2C wealthy countries will have to cut their emissions by somewhere in the neighbourhood of 8 to 10% a year5.
Greenhouse gases and their link to fossil fuels
Carbon dioxide is the greenhouse gas that is most commonly produced by human activities and it is responsible for 64% of man-made global warming. Its concentration in the atmosphere is currently 40% higher compared to pre-industrialisation levels. Other greenhouse gases are emitted in smaller quantities, but they actually trap heat far more effectively than CO2: methane is responsible for 17% of man-made global warming and nitrous oxide for 6%6.
Other greenhouse gases include water vapour, which despite being the most abundant of these gases does not produce the same effect as its level in the atmosphere depends on temperatures, and chlorofluorocarbons (CFCs) which are now one of the most heavily regulated industrial chemicals after it was discovered they were contributing to the destruction of the ozone layer.
There is a variety of activities that contribute to the release of greenhouse gases. Carbon dioxide and nitrous oxide are released by burning coal, oil and gas. Even though oil is considered one of the dirtiest fossil fuels, it still is the main global energy source and it is responsible for producing 46% of global carbon dioxide emissions, according to data gathered in 20037.
While our industrial activities, energy generation and transport are major emitters of greenhouse gases, there are other ways in which humans have contributed to the increase of greenhouse gases. For example, cutting down forests means that the carbon stored in trees is release into the atmosphere while we can no longer benefit from their ability to act as carbon sinks. Similarly, livestock farming, such as cows and sheep produce large amounts of methane when they digest their food. And fertilisers used in agriculture produce nitrous oxide emissions6.
There is a lot of scientific evidence supporting the case that human activity is contributing to the kind of climate change we are experiencing today. Understanding the basic science behind climate change is key information in today’s world. Informed citizens are critical to tackling this issue with the right policies.