The climate change we are experiencing today is driven by human activities that increase greenhouse gas concentrations in Earth’s atmosphere. Although we mainly hear about greenhouse gases in the context of their contribution to climate change, they are in fact essential for life on Earth. Consider for a moment carbon dioxide’s (CO2) critical role in photosynthesis, and water vapor’s role in the formation of rain. Both of these gases are greenhouses gases. Greenhouse gases earn their name because they function much like the glass covering a greenhouse; they allow sunlight to easily pass through the atmosphere but trap the reflected heat energy so that it stays close to Earth’s surface. This greenhouse effect allows all the organisms on Earth, even us humans, to flourish. Without greenhouse gases, temperatures would drop, and our planet would be too cold to sustain life. However, high concentrations of greenhouse gases can also be harmful. Think for a moment of greenhouse gases as “blankets” covering the Earth’s surface: more “blankets” will trap more heat, giving rise to higher temperatures. This is exactly what is happening today—human activities are currently increased greenhouse gas concentrations in the atmosphere so much, and at such a fast pace, that Earth is heating up too fast for biodiversity to adapt to the changes.
Greenhouse gases are essential for life on Earth. But too much of them cause Earth to heat up too much too fast, leading to climate change.
At present, the single biggest cause of increased greenhouse gas concentrations is the burning of fossil fuels. Since the Industrial Revolution about 200–250 years ago, humans have become heavily dependent on the energy captured in these fuels—coal, oil, and natural gas—for activities such as transportation, heating, manufacturing, and electricity generation. Fossil fuels contain a high percentage of carbon, so when it is burned, that carbon is released into the atmosphere, generally as CO2. Consequently, since human populations started exploding and have been using fossil fuels at increased rates, the greenhouse effect has been significantly amplified.
While fossil fuel burning is currently the biggest overall driver of climate change, the destruction of carbon sinks, such as tropical forests (Box 10.3.1) and peatlands is also a large contributor. Destroying these ecosystems contributes to rising atmospheric CO2 concentrations directly through burning of vegetation that releases carbon, and indirectly through the loss of vegetation that would otherwise extract CO2 from the atmosphere if they were still alive. The contribution of ecosystem loss to climate change is substantial: 13% of today’s global carbon emissions can be accounted for by tropical deforestation (IPCC, 2014).
The link between human-induced climate change and atmospheric CO2 concentrations was first highlighted in the late 19th century (Arrhenius, 1896). However, it was not until the mid-1950s (e.g. Kaempffert, 1956) that scientists started to raise concerns about increasing CO2 concentrations in the atmosphere. By the 1980s, as global annual mean temperatures started to rise, consensus about climate change linked to CO2 began to spread among the broader public. Yet concrete steps to curb CO2 emissions would only be initiated decades later. In the meantime, CO2 emissions continue to accelerate (Figure 10.3.2): more than 37 billion tonnes of carbon, a new record, were released into the atmosphere in 2018 (Jackson et al., 2018; Le Quéré et al., 2018). To put it in another way, during 2018, humans released on average over 100 million tonnes of CO2 into the atmosphere every day.
The second-most important greenhouse gas that drives climate change is methane (CH4). Methane is a natural by-product emitted from decaying organic matter, most notably from wetlands that inhibit the speed of decomposition. These important ecosystem processes release methane into the atmosphere, albeit in relatively low concentrations. However, human activities have boosted methane emissions significantly over the past few centuries, through wasted food decaying at landfills, leaks from natural gas wells, an increase of industrial-scale cattle and dairy farms, and large-scale destruction of swamps and peatlands. Warmer temperatures also result in the drying of wetlands and peatlands; this drying speed up decomposition of organic material, which increases the rate of methane release. Methane currently constitutes 16% of all global greenhouse gas emissions released by humans (IPCC, 2014). This may not seem to be a major contribution; however, methane is 72 times more effective than CO2 in trapping radiation over a 20-year period (Forster et al., 2007), so even small increases in atmospheric methane can have dramatic effects.
The third important greenhouse gas that drives climate change is nitrous oxide (N2O), also known as laughing gas. Nitrous oxide is a by-product of synthetic fertilizers used in agriculture, burning of fossil fuels, and several industrial processes, and accounts for 6% of all human-caused greenhouse emissions (IPCC, 2014). However, it is even more potent than methane, and stays in the atmosphere for about 114 years, so the impact of one tonne of N2O is equivalent to 310 tons of CO2 over 100 years (Forster et al., 2007).