Human evidence: Matching temperature trends
Scientists have calculated that a combination of increased solar activity and decreased volcanic activity contributed to some of the global warming observed in the first half of the 20th century. But even a combination of all known natural factors can’t account for the warming of about 0.5 °C observed since 1970. However, when scientists included the effects of human activities in their calculations, they were able to account for the measured trend in global temperature within the uncertainties.
Volcanic activity and solar variations affect climate. Scientists monitor both and calculate their influence on global temperatures. Overall, volcanic eruptions tend to have a cooling effect for a year or so because of the aerosols they eject into the atmosphere, which reflect some sunlight. In contrast, solar variations can have warming or cooling effects depending on energy output in the different wavelengths of solar radiation. Climate models indicate that while increasing solar activity and decreasing volcanic activity – in combination with ‘natural variability’ (such as El Niño) – contributed to some of the warming observed over the first half of the 20th century, they can’t account for the measured warming trend observed since 1970.
Human use of fossil fuels, deforestation and changes in land use cause emissions of greenhouse gases and black carbon, which have a warming effect. But human use of fossil fuels also results in emissions of aerosols, which have a cooling effect. Using computer models, scientists calculate that human activities have been having an overall warming effect which has increased over the 20th century, especially in the last four decades. Combining the effects of human activities with known natural factors, such as solar and volcanic changes, has enabled scientists to create computer models that can simulate – within certain limits – both the amount and the rate of warming since 1970.
Emissions of carbon dioxide (CO2) and other greenhouse gases from human activities accelerated over the 20th century. In the 1950s, CO2 emissions from fossil fuel burning amounted to just 6 gigatonnes (a gigatonne is a billion tonnes), but this rose to nearly 30 gigatonnes in 2009. About half of the extra CO2 has accumulated in the atmosphere, and will raise the level of CO2 in the atmosphere for many centuries. This build-up of CO2 causes a warming effect which is increasing year on year.
In the 1950s greenhouse gas emissions from human activities began to increase sharply. However, at this time humans also emitted large quantities of aerosol particles. These blocked out some sunlight, leading to a cooling effect at the Earth’s surface. Scientists have calculated that the cooling effect of human aerosol emissions, though uncertain in magnitude, temporarily offset the warming effect of greenhouse gas emissions to a significant extent. However, health concerns led governments in the technologically advanced nations to introduce measures to reduce pollution, markedly decreasing aerosol emissions. Scientists’ calculations suggest that the warming effect of greenhouse gas emissions has now begun to outweigh the cooling effect of aerosol emissions – and this imbalance is increasing.
It’s not always easy to spot a change in the climate. Sometimes it takes someone like Nikos Christidis to really examine the numbers. Nikos analyses data from climate measurements and models to identify changes in the climate and assess their possible causes.
Nikos’s work has played a part in establishing that human influences have played a prominent role in the warming observed in recent decades. He’s also established that extremely warm nights and days are becoming more common as a result of human influences. ‘It’s nice to know my work is informing policy and could even help society adapt to climate change,’ says Nikos.