Over the last two centuries, the growth of fossilfuel combustion has been closely coupled to global growth in energy use and economic activity.
Fossilfuel emissions grew by 3. Although fossil-fuel emissions of CO 2 have grown fairly steadily, the upward march of the CO 2 concentration in the atmosphere varies from year to year. This is caused mainly by the effects of weather variability on vegetation, and also by sporadic volcanic activity: major volcanic eruptions have a significant indirect influence on atmospheric CO 2 concentrations, causing temporary drawdown of CO 2 through the promotion of plant growth by the light-scattering and cooling effects of volcanic haze.
Climatic warming or cooling arises from changes in the flows of energy through the climate system Figure 1. The main drivers that have acted over the last century are:. Of these, solar fluctuations and volcanic eruptions are entirely natural, while the other four are predominantly caused by human influences.
The human-induced drivers have been dominant over the past century Figure 3. Changes in greenhouse gas concentrations, dominated by CO 2 , caused a large warming contribution. Some of this has been offset by the net cooling effects of increased aerosol concentrations and their impact on clouds.
Black carbon or soot has probably exerted a smaller, warming influence. The net effect of all aerosol types including soot remains hard to quantify accurately.
Among the natural influences, the effect of changes in the brightness of the Sun has been very small Box 3. Volcanic influences are highly intermittent, with major eruptions such as Pinatubo in causing significant cooling for a year or two, but their average effects over the past century have been relatively small.
In comparison with other influences, the effects of solar variations on present global warming are small. At present, humans are putting an estimated 9. Of this human-produced carbon, forests and other vegetation absorb around 3. A net 5 billion metric tons of human-produced carbon remain in the atmosphere each year, raising the global average carbon dioxide concentrations by about 2. Since , humans have increased the abundance of carbon dioxide in the atmosphere by nearly 50 percent.
Learn more. Fahey, K. Hibbard, D. Dokken, B. Stewart, and T. There is abundant evidence for mass extinctions of life during these periods. And climate models suggest that, at times, the tropics could have become "dead zones" , too hot for most species to survive.
For many years, groups of so-called climate "sceptics" have cast doubt on the scientific basis of global warming.
However, virtually all scientists who publish regularly in peer-reviewed journals now agree on the current causes of climate change. A key UN report released in said it "is unequivocal that human influence has warmed the atmosphere, oceans and land".
The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.
Top image credit: Getty Images. How do we know the world is getting warmer? Image source, Reuters. Turkey was one of the places hit by devastating wildfires this summer. How do we know humans are responsible for global warming? Image source, Getty Images. What does net zero mean? Is the UK on track to meet its climate targets? The number of weather-related disasters has increased by a factor of five over 50 years.
What impact are humans having on the planet? But wasn't it warmer in the past?
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