Mitigation and adaptation are the two main approaches in addressing the climate crisis. Mitigation seeks to prevent further temperature rises by reducing anthropogenic greenhouse gas emissions, while adaptation seeks to acclimatise societies to a world impacted by current climate change. Given the impending impacts of more severe global warming, governments cannot afford to slack off in formulating mitigation policies. However, even if all greenhouse gas emissions were stopped today, would global warming come to an immediate standstill? 

The climate crisis is humanity’s greatest challenge, even as the world’s attention is currently on the COVID-19 pandemic. Of the culprits that drive climate change, anthropogenic carbon dioxide is one of the most directly culpable. A greenhouse gas, carbon dioxide absorbs infrared radiation emitted from Earth’s surface and releases it back to Earth’s surface, contributing to atmospheric warming. Other greenhouse gases include methane, nitrous oxide and ozone. Not all emissions have a warming effect, however; sulfur oxides, for example, produce cooling.

Collectively, these substances are referred to as anthropogenic climate forcers, because they are human-induced factors that force the climate warmer or cooler by altering the flow of energy into and out of Earth. The current climate crisis is the product of exerting such climate forces on the global environment – industries releasing massive plumes of emissions into the atmosphere since the 1970s, alongside man-made projects that tremendously transformed natural landscapes, altering the degree of sunlight reflection by land.

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So how do climate forcers cause climate change? To put it simply, the amount of radiation the Earth absorbs from the sun must match the amount of radiation the Earth emits back to space. The difference between incoming and outgoing radiation is referred to as radiative forcing (RF). Positive RF occurs when outgoing energy is less than incoming energy, producing a warming effect. Conversely, negative RF occurs when outgoing energy is greater than incoming energy, producing a cooling effect.

Evidently, anthropogenic climate forcers have been primarily warming the planet. Since the Industrial Revolution, global temperatures have increased by about 0.2°C per decade. Global surface temperatures in 2019 were about 1.1°C warmer than in 1880. This trend of temperature rise is expected to continue on for several decades, no matter the levels of anthropogenic emissions.

The impacts of the climate crisis are far-reaching, sparing no one. They include more frequent and intense weather phenomena, greater sea-level rise, threatened ecosystems, food insecurity and adverse health effects. The urgency of climate mitigation led to the formulation of the Paris Agreement in 2015. Its objective is clear: to keep global warming to well below 2°C above pre-industrial levels by 2100, while pursuing a limit of 1.5°C global temperature increase. Achieving this entails halving global anthropogenic CO2 emissions by 2030 and reaching net zero by 2050.

To achieve these ambitious goals, all countries need to come together and engage in a substantial and sustained effort towards reducing anthropogenic climate forcing emissions. However, as powerful mitigation efforts may ‘[entail] substantial costs and short-term perceived burdens on society’, their benefits should be publicly elucidated in order to gain and retain public support.

However, recent findings by scientists show that it will take decades for today’s mitigation policies to produce tangible outcomes on global warming. These findings reinforce conclusions of past research. Strong, sustained mitigation efforts notwithstanding, a delayed climate response ought to be expected, with global temperatures taking decades to be reduced perceptibly.

What would happen if greenhouse gases disappeared?

Investigating delayed detections of climate mitigation benefits arising from individual and combined reductions of climate forcing emissions, the scientists’ simulations demonstrated that if we fully stopped greenhouse gas emissions in 2020, warming could be halted as soon as 2033, but this is highly impractical – essentially an overnight switch away from all carbon-emitting energy sources, including fossil fuels which provide 80% of the world’s energy.

If global warming persisted until 2033 even if we stopped greenhouse gas emissions starting this year, what kind of scenario would arise from an annual 5% reduction of CO2 emissions starting 2020? It turns out that another 10 years is likely added to the delayed detection; global warming would start slowing down only in 2044. Nonetheless, even a 5% reduction is a tall order. To put things in perspective, it took a pandemic and the resulting widespread economic shutdown to bring a 4-8% reduction in global CO2 emissions. Reaching the limit of 1.5°C temperature increase requires an annual 7.6% emission reduction in the current decade. As economics eventually recover, emissions may easily bounce back up to pre-COVID levels if governments do not begin or resume a commitment to carbon mitigation. 

There are multiple reasons for the delayed effectiveness of emission reductions. First, the internal variability of the climate. The flow of energy in and out of Earth, and by extension the Earth’s climate, does not solely depend on anthropogenic climate forcers like human-induced emissions. Natural climate forcers play a part as well, such as phenomena like El Nino, La Nina and volcanic activities. Depending on their phase, they could serve to neutralise or intensify the effects of anthropogenic emissions. Hence, the interplay between natural and anthropogenic climate forcers can explain why the effectiveness of emission reductions would take decades to emerge.

Even If Emissions Stopped, Carbon Dioxide Could Warm the Earth for Centuries

Second, carbon dioxide is a very long-lived gas, having a lifespan of hundreds of years, possibly up to a thousand. The chief culprit of human-induced global warming, past and current anthropogenic CO2 emissions will end up lurking, accumulating in the atmosphere for decades, even centuries, contributing to long-term global warming, before being reabsorbed by either natural carbon sinks or artificial means. Other industrial emissions, such as methane and tropospheric ozone, have shorter atmospheric lifespans, thus exerting shorter effects on the climate. Would clamping down on these short-lived gases result in faster emergence of climate mitigation benefits? Unfortunately, the scientists found that even though methane is 80 times more potent than carbon dioxide on a 20-year timescale, reducing methane emissions to zero starting this year would only slow warming from 2039. Furthermore, while strong mitigation of short-lived climate forcers like black carbon (a component of particulate matter formed from incomplete combustion of fossil fuels) may see earlier emergence of climate mitigation benefits than carbon dioxide mitigation, the eventual accumulation of continued CO2 emissions is likely to offset these benefits in the near future. As phrased by the scientists, ‘targeting [black carbon] emissions… would be efficient- however, with a low final payoff’.

While these findings can be rather dismaying, mitigation still remains key to achieving the goals of the Paris Agreement. What is crucial now is to clearly communicate these scientific findings to the public to manage expectations and maintain support for mitigation policies along the years. While the scientists recommend using atmospheric greenhouse gas concentrations to assess the effectiveness of mitigation policies in order to remove the confounding effect of natural variability, temperature still remains the most intuitive indicator of climate change.

To illustrate the risk of insufficient scientific communication, if all greenhouse gas emissions were to be completely stopped starting this year, the public would expect immediate, rapid declines in global warming. However, temperatures are projected to rise at an accelerated pace for at least 13 more years. Without careful, nuanced media messaging to convey and explain this projection, the lack of immediate outcomes may lead to public backlash on the perceived ineffectiveness of the policies, and sustaining climate mitigation efforts in the long-run could be met with significant pushback.

Ultimately, pushing for strong, sustained mitigation efforts to counter the climate crisis will require the collective cooperation between governments, the scientific community and the media.

Featured image by: United Nations Photo