How Net Zero Became Our Global Climate Goal And Why We Need It

2021 was the year net-zero commitments went mainstream. Following the COP 26 climate conference in Glasgow, well over 100 governments have set a net-zero decarbonization target. These commitments cover 88% of global emissions, 90% of global GDP, and 85% of global population. Nearly one-thousand municipalities and over three thousand private businesses have also made net-zero commitments as part of the UN’s Race to Zero decarbonization initiative.

While these commitments are a positive first step, they must now be backed by concerted action. The stakes are higher than ever as climate change continues to intensify. Net zero by 2050 is a science-based goal that aims to limit the worst effects of global warming. Policymakers, business leaders, and the public should understand both the origins and urgency around this crucial climate goal.

A manmade crisis

Human activities produce around 35 billion tons of carbon dioxide annually. When land use changes (e.g. clearing forests for ranching) and other greenhouse gases (e.g. methane, nitrous oxide) are included, total human-caused (termed “anthropogenic”) emissions are equivalent to over 50 billion tons of carbon dioxide. The concentration of carbon dioxide in the atmosphere has risen 50% since pre-industrial times, from around 280 parts per million (ppm) to almost 420 ppm (other greenhouse gases have also increased). The rising concentrations of greenhouse gases are warming the planet at an accelerating pace. The Intergovernmental Panel on Climate Change (IPCC), the world’s leading authority on climate change, reported that Earth is now 1.1˚ C warmer than in pre-industrial times and “climate change is already affecting every region on Earth.” Damaging climate impacts, from worsening heatwaves to rising seas and from stronger storms to more frequent wildfires will intensify with additional warming.

Global awareness of climate change has existed for decades. The IPCC was founded in 1988 and released its first assessment report on climate change in 1990. In 1992, at the Rio Earth Summit, world leaders agreed to the United Nations Framework Convention on Climate Change (UNFCCC), which convenes the annual Conference of the Parties (COP) to agree on international climate action. A few years later, the Kyoto Protocol was signed, pledging nations to limit their greenhouse gas emissions. Despite such agreements, global emissions continued to rise rapidly. Indeed, over half of anthropogenic emissions have been produced just since 1990.

Worsening climate-related disasters have become impossible to ignore, and climate has become a central concern for communities across the globe. The dire forecasts for a warming world have led to growing calls for climate action. 2015 saw nearly 200 nations sign the landmark Paris Agreement, the most ambitious climate accord to date. The Paris Agreement commits to “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C,” to mitigate the worst impacts of climate change.

Into the danger zone

When the Paris Agreement was signed, many climate scientists considered 2˚ C of warming a safe threshold below which climate change would be disruptive but manageable, and above which the risks of catastrophic and irreversible climate change would be unacceptably high. However, in the years that followed, improved observational data and powerful computational models offered alarming evidence that even “moderate” global warming (~2˚ C) could have dangerous consequences. The IPCC compiled this emerging research in its Special Report on 1.5˚ C. This report demonstrated that potential impacts of 2° C of warming would be significantly more severe than 1.5˚ C with millions more facing water scarcity and heatwaves, lower agricultural yields and less nutritious crops, and more creatures facing extinction including nearly all coral reefs.

Perhaps most concerning, climate science has shown that complex natural systems are far more sensitive to global warming than had been previously thought. Within these systems exist tipping points: points at which the climate can swing into a radically new state. Such tipping points include the collapse of major ecosystems like the Amazon rainforest, the melting of the Greenland and West Antarctic ice sheets, the thawing of permafrost, and the destabilization of ocean currents. Many tipping points are subject to positive feedbacks that enhance their effects and are also interrelated. Consider how rising temperatures melt highly reflective ice, leaving darker water and rocks to absorb even more heat, melting even more ice. This influx of meltwater can then impact the flow of ocean currents by changing the temperature and salinity of the surrounding water.

Research from Exeter Professor Tim Lenton sounded the alarm on climate tipping points, with “potentially irreversible changes in the climate system [are] under way, or very close.” New analysis of paleoclimates is revealing how rapidly these tipping points can be reached along with their cataclysmic effects. The impacts of subtle temperature shifts can be profound. Within the past million years, at temperatures similar to those today, Greenland was completely ice free. During the most recent ice age, which saw much of the Northeastern U.S. covered under a mile of ice, temperatures were only 5˚ C lower than at present. As scientists continue to warn, when it comes to climate change, “uncertainty is not our friend.”

Turning down the heat

While no one can be certain precisely how and when tipping points may manifest, fundamentally, climate action is an exercise in risk management. Given the catastrophic potential of uncontrolled warming, 1.5˚ C has become widely accepted as a critical threshold for planetary safety. If humanity continues to emit greenhouse gases into the atmosphere, the planet will soon warm past this threshold. Fortunately, climate science indicates that reaching net-zero carbon dioxide emissions and declining levels of other greenhouse gases would halt global warming within decades.

A commonly used bathtub analogy helps explain the emissions challenge. The water in the bathtub represents emissions in the atmosphere and the water from the tap represents new emissions. The drain represents natural carbon sinks like forests and human activities like carbon capture, all of which remove emissions from the atmosphere. The drain can be increasingly clogged by cutting down forests and overburdening the oceans with carbon dioxide. If the tap runs faster than the tub drains, the water level rises. In this analogy, when the water reaches the top of the tub, temperature rise will likely exceed 1.5˚ C. Considering the drain, the quantity of water that can come out of the tap (the new anthropogenic emissions) before the tub overflows represents the remaining carbon budget. However, if the rate of the tap and the rate of the drain are balanced, the water level will no longer rise. This steady state is called net zero. Likewise, the IPCC defines net zero carbon dioxide emissions as “when anthropogenic carbon dioxide emissions are balanced globally by anthropogenic carbon dioxide removals over a specified period.”

Understanding how much remains of the carbon budget and when the world needs to reach net-zero carbon dioxide emissions has tremendous policy, economic, and development implications. The IPCC assessed numerous climate models and indicated that for a 67% chance to stay below 1.5˚ C of warming, only 400 billion tons of carbon dioxide can be added to the atmosphere. For reference, that represents around 8-11 years of annual global emissions. With uncertainties around climate sensitivity, lower carbon budgets imply higher probabilities of hitting the 1.5˚ C target but require emissions to fall further and faster.

Another IPCC working group (WG 3) explored mitigation scenarios that allow the world to stay within its carbon budget. Scenario modelers consider the economic, societal, technological, and policy factors needed to limit global warming to 1.5˚ C. This work attempts to incorporate realistic feasibility considerations into emissions reduction pathways. A metanalysis of 1.5˚ C emission reduction pathways demonstrated that on average these scenarios see carbon dioxide emissions fall by 45% from 2010 levels by 2030 and reach net zero by 2050. Thus, the date of 2050 for net zero reflects assumptions around how quickly and effectively the global economy and energy systems can be decarbonized.

Running out of time

Net zero by 2050 has only recently become an acknowledged climate goal for many nations and corporations. However, this ambitious target is already under threat. As the IPCC wrote in its latest assessment report, “unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming to close to 1.5° C or even 2° C will be beyond reach.” The scale of the challenge is daunting. Carbon dioxide emissions need to fall by approximately 7% annually until 2030 to keep the world on track for 1.5˚ C. By comparison, 7% is how much the chaos of the COVID-19 pandemic reduced emissions in 2020. While it is a hopeful sign to see that major global actors have made net zero by 2050 pledges, commitments without action are just talk.

Subsequent articles will explore net-zero commitments to help separate the green from the greenwash and consider the implications of different net zero trajectories.