What are the benefits of reducing global <scp>CO₂</scp> emissions to net‐zero by 2050?

Abstract

This is where total global anthropogenic carbon dioxide (CO2) emissions are reduced to near zero, and any remaining emissions are balanced by measures to remove CO2 from the atmosphere (Allen et al., 2018). Halving all CO2 emissions by 2030 and achieving net-zero emissions by 2050 is anticipated to meet the aspiration of the Paris Agreement by limiting the global mean surface temperature increase to 1.5 degC above pre-industrial values, as well as avoiding some of the worst impacts from climate change and reducing the probability of reaching dangerous climate tipping points. Emissions of other greenhouse gases (such as methane) and black carbon have a shorter lifespan in the atmosphere, but are nevertheless important to reduce alongside CO2 emissions to achieve the 1.5 degC temperature target. While the Earth's climate system will still experience year-to-year variability, achieving net-zero and limiting warming to 1.5 degC, as opposed to 2 degC, above a baseline of 1850–1900, will reduce the frequency and intensity of weather events such as heatwaves, heavy rainfall and droughts (Allen et al., 2018). The IPCC Special Report on the impacts of global warming of 1.5 degC (Allen et al., 2018) (SR1.5) concluded with high confidence that extremely hot days are projected to be 3 degC warmer at global warming of 1.5 degC, and 4 degC warmer at 2 degC, with the largest increases in the number of hot days projected to be in the tropics. An additional 0.5 degC global warming compared to the present is associated with more intense and more frequent heavy rainfall in several regions, leading to an increase in the global land area affected by flood hazards. Limiting global warming to 1.5 degC is projected to reduce sea level rise by around 0.1m compared to 2 degC by 2100, although sea level will continue to rise well beyond 2100 (Allen et al., 2018). Based on 2010 population estimates, this would result in up to 10 million fewer people exposed to sea-level related risks. While many climate impacts are observable in the current climate, large-scale singular events (such as Greenland ice sheet collapse, collapse of North Atlantic circulation or major global forests becoming sources of CO2 rather than sinks) have a low probability, but potentially catastrophic, irreversible consequences. These tipping points are important because reaching them may further exacerbate projected impacts (e.g. West Antarctic ice sheet collapse would lead to 3–4m of sea level rise (Bamber et al., 2009)) and would accelerate warming leading to cascading impacts (Rocha et al., 2018), increasing the probability of multiple tipping points being reached (Lenton et al., 2019). Achieving net-zero CO2 emissions by 2050 and limiting global warming to 1.5 degC would reduce the probability of the most damaging effects of climate change on natural systems. However, even at 1.5 degC global warming there is a severe risk of significant local change to water resources and terrestrial ecosystems in the Arctic and some sub-tropical to temperate dry zones (Gerten et al., 2013). The SR1.5 concluded that 13% of the global land area was projected to undergo ecosystem transformation at 2 degC global warming but limiting warming to 1.5 degC could reduce this area by 50%. In the oceans, limiting global warming to 1.5 degC reduces risks of severe impacts on ocean carbon uptake, coral reefs, bivalve fisheries and coastal protection amongst other impacts (IPCC, 2018). However, it does not avoid the very high risk of significant irreversible impacts to warm water corals due to the combined effect of warmer oceans and acidification from higher levels of dissolved inorganic carbon. As net-zero would be achieved by reducing the burning of fossil fuels, we can expect to see direct and immediate benefits to human health due to reduced air pollution, particularly in urban areas. Additionally, the measures we take to achieve net-zero may also benefit human health. For example, by encouraging active travel such as cycling, we would expect to have a healthier population with reductions in respiratory illnesses, fewer traffic deaths, and reduced noise pollution. Similarly, where diets have more red meat than is recommended by healthy eating guidelines, replacing red meat with pulses and other vegetables would both reduce associated greenhouse gas emissions and improve health outcomes. Achieving net-zero may also reduce the severity of regional hydrometeorological events such as drought and flooding due to heavy rainfall. Drought events can affect water quality and availability, food production and increase risks of wildfires as well as decreasing land carbon sequestration. Limiting warming to 1.5 degC may reduce the global land area under aridification (a long-term increase in dry and hot conditions) by up to two thirds (Park et al., 2018). Achieving this target may also help to limit risks of flooding in major catchments around the world (Alfieri et al., 2017). Achieving net-zero CO2 by 2050 and consequently stabilising global mean temperatures at approximately 1.5 degC above preindustrial levels would avoid some of the worst impacts of climate change predicted at 2 degC and above. These benefits are likely to be experienced in most natural and human systems. Furthermore, the co-benefits of achieving net-zero in terms of a healthier population are also an important consideration.

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