In the race to stabilise the climate, net zero is the finish line. How quickly we get there determines the temperature we reach, and the lower the better. But there’s no escaping the race if we want a viable, habitable and investible future. Net zero is non-negotiable, explains Dr Sam Cornish, IIGCC Transition Research Specialist.
Politics and semantics change: physical fundamentals do not. In the context of some policy backsliding on climate, and the hottest two years on record, we spoke to IIGCC’s Dr Sam Cornish to clarify the fundamental significance of net zero for our Earth’s climate.
Dr Sam Cornish: No. Net zero is a necessary condition for warming to stop – whether this is at 1.5°C or at any level. Net zero does not specify the temperature rise itself.
Temperatures will rise indefinitely until we reach net zero. So, if humans want to live on a safe and habitable planet, net zero is imperative. There is no credible safe future for humanity on a planet that warms forever.
Net zero is the point at which anthropogenic (human-made) emissions of carbon dioxide (CO2) are balanced by anthropogenic removals of the gas. Due to the difficulty of removing CO2 from the atmosphere and storing it long-term, net zero implies very low gross emissions. We’re usually speaking about net-zero CO2 owing to its significance in halting warming, but we can also talk about net-zero greenhouse gases (GHGs), which is a different thing.
The total amount of CO2 we emit between now and reaching net zero will determine the peak temperature, as well as the emission rates of short-lived substances like methane at the time.
CO2 is a long-lived gas in the atmosphere – once it’s there, it’s hard to get rid of. Much of what we emit today will remain for thousands of years. Scientific observations, modelling and theory indicate that temperatures go up linearly with cumulative emissions of CO2. This leads to the concept of a ‘carbon budget’ for a given temperature outcome (Fig. 1).
Figure 1: Carbon budgets for different warming levels. Data from Lamboll et al. 2023
It’s the total amount of cumulative CO2 emissions that we think can be emitted, or ‘spent’, for a certain percentage likelihood of achieving a given peak temperature outcome. The likelihood is estimated using many models (numerical simulations) of the climate. The budget for 1.5°C is significantly smaller than for 2°C, as Figure 1 shows.
Net zero, as correctly defined, does not include natural sinks of CO2 that are ‘passive’ and involve no human intervention (Fig. 2). There is a danger that including these sinks in accounting, as some countries have done, will artificially flatter emissions budgets, and lead to higher warming outcomes.
Figure 2: Defining net zero
That’s right, they’re not the same. Properly defined, reaching net zero implies a state of slowly decreasing CO2 concentrations. In the absence of additional emissions from human activity, passive natural sinks of CO2 very slowly remove the gas from the atmosphere. The distinction is important because the world would continue warming if concentrations were merely held constant.
The current scientific consensus is that when we reach net zero, warming will stop, within ±0.2°C. This is because the slow thermal equilibration of the Earth is roughly balanced by the slow decrease in CO2 concentrations.
Unlike CO2, short-lived gases are removed naturally from the atmosphere on relatively short timescales and so their warming effect is related to emission rates. We need emissions of these gases to shallowly decline to stabilise their warming effect. However, much deeper cuts would dramatically improve our likelihood of limiting warming to 1.5°C or 2°C, as it would begin to reverse the warming we have experienced from these gases so far.
Net zero was enshrined in the Paris Agreement and reiterated in subsequent UN Conferences of the Parties (COPs). Article 2 of the Paris Agreement set out a commitment to ‘holding the increase in the global average temperatures to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C.’
To achieve this, it was agreed in Article 4.1 that parties would aim ‘to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century.’ More recently, the UAE Consensus reached at COP28 called on parties ‘to achieve net zero by 2050 in keeping with the science.’
Yes. When we sum all greenhouse gases we include short-lived gases like methane and hydrofluorocarbons alongside CO2. We aggregate using factors called global warming potentials that are useful for accounting but do not replicate the declining effect of these gases over time. As such, the climate implications of the two ‘net zeros’ are different, as clarified by the IPCC since the Paris Agreement.
Net zero CO2 is needed to halt warming, while net-zero GHGs implies declining temperatures and happens decades later than net zero CO2 in most climate scenarios.
Figure 3: Net zero GHGs occurs decades later than net zero CO2 in IPCC climate scenarios.
The Paris Agreement ambition to limit warming to 1.5°C recognised that this would ‘significantly reduce the risks and impacts of climate change,’ and subsequent UN climate conferences have reiterated the 1.5°C goal.
While there is no special physical status to this precise threshold, we do know that with every increment of warming, the greater the impacts, with significantly worse outcomes in a 2°C world compared to a 1.5°C world.
Another key consideration is that the likelihood of crossing ‘tipping points’ increases after passing 1.5°C, with Amazon dieback and irreversible Greenland ice sheet loss at risk between 1.5°C and 2°C.
Net zero is a physical concept. There is scientific consensus regarding its importance for stabilising temperatures. That said, how we get there is operationalised through social, political and economic systems. But if we want a safe and habitable planet to live and do business on, it is a non-negotiable destination.
Thank you, Sam.
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