🔗Climate recap

🔗Climate recap: key concepts

[Updated Dec 2022]

This page is a recap of how greenhouse gases from human activities are accumulating in the atmosphere and trapping heat, thereby causing the increase in global temperature. To stop the temperature rising, therefore, we have to stop the gases accumulating. To do this we need to balance the quantity of gases released with the quantity removed from the atmosphere. Based on past emissions and the rise in temperature, we can estimate by how much, and how quickly, we need to reduce emissions to avoid the worst effects of global warming.

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🔗Earth’s temperature is rising ▼

The global average temperature of the earth has been steadily rising. It has now increased by around 1.2°C above the pre-industrial period 1850-1900, see Figure 1.

In the 2015 Paris Agreement 195 nations agreed to try to limit the temperature rise to 1.5°C.3 This target of 1.5°C is there to avoid the more severe impacts that would be incurred by greater temperatures.4

🔗Greenhouse gases from human activities raise the temperature ▼

The rise in temperature is attributed to changes in greenhouse gases in the atmosphere, largely as a result of human activity.5 The main change comes from the burning of fossil fuels, such as coal, oil and gas, which release the gas carbon dioxide (CO2). However the increase in methane (from activities such as livestock farming, rice cultivation and coal and gas production) is also contributing to warming.6 Although methane is short-lived, it is much more potent than carbon dioxide. The IPCC note that reduction of short-lived, potent greenhouse gases, such as methane, can play a role in short-term control of temperature.7

🔗How greenhouse gases contribute to global warming ▼

The gases that accumulate in the air allow light from the sun to pass through to warm the earth. But when it is radiated back to the atmosphere as heat, it is trapped by the gases in our atmosphere causing the temperature to rise. The glass in a greenhouse traps heat in the same way, hence the name greenhouse gas.9

🔗Balancing emissions and removals for zero net emissions ▼

The gases in the atmosphere have built up because the rate of releasing them into the atmosphere exceeds the rate at which earth systems can remove them. Parts of earth systems that remove gases are called sinks, and examples of these are the oceans, trees and soils.

As mentioned earlier, it is the gases from human activities that are producing the imbalance and enabling them to accumulate. So to stop the temperature of the earth rising, we need to balance the greenhouse gases that are released with the greenhouse gases that are removed. This balance is carbon neutrality, where there are zero net emissions, often referred to as ‘net zero emissions’, and abbreviated to ‘net zero’.

🔗Estimating a Carbon Budget ▼

Since we want to limit the earth’s average increase in temperature to around 1.5°C, and since it is our emissions that are raising the temperature, we need an estimate of the upper limit of emissions that would be consistent with the 1.5°C threshold.

This is what is called our carbon budget. It is a budget specifically for carbon dioxide (CO2) emissions. So it does not address the reductions needed to remain within the 1.5°C threshold for other non-CO2 emissions, such as methane.

The available carbon budget, and allowable remaining global warming, is limited by an estimate of non-CO2 gases and other substances that are expected still to be in the air when we reach net zero. Since these will contribute to remaining warming at the time of net zero, it reduces the allowable warming from carbon dioxide, and hence the budget.10

We know from past emissions that for each unit of cumulative CO2 in the atmosphere, the average global temperature increases by a roughly proportional amount (this relationship is called the TCRE).

So we can use this relationship to estimate the limit on our CO2 emissions. For example, if the temperature is estimated to increase by around 0.45°C for every 1000 GtCO2 emitted, and if we have an allowable warming of 0.4°C, then we can divide by 0.45°C to get the number of gigatonnes of CO2 for our carbon budget: (0.4/0.45)*1000=889 GtC02. If on the other hand the temperature rises by 0.7°C for every 1000 GtCO2, then the carbon budget would be smaller (around 571 GtCO2).

Likewise, as our remaining available warming reduces, our remaining budget falls. For example, an available remaining warming of 0.15°C with a warming rate of 0.45°C for every 1000 GtCO2 gives a budget of 333 GtC02 (0.15°C/0.45°C)*1000).

🔗Our remaining carbon budget

In 2020 the Intergovernmental Panel on Climate Change (IPCC) estimated that we had a carbon budget of 400 GtCO2 from 2020 to have a 2 in 3 chance (66%) of staying within our target.12

However, there are many uncertainties relating to these calculations, which the simplified description in the previous section did not reflect. These uncertainties could lead to either an underestimate or an overestimate of the remaining budget

For example, there are varying estimates of the non-CO2 contribution to global warming at the time of net zero, which could change the budget by ±220 GtCO2. Uncertainties about the extent of warming so far could add or subtract another 550 GtC02.

There is also an assumption that large reductions in non-CO2 gases, such as methane, will be made before the time of zero net emissions. If the reductions are not made, the non-CO2 warming contribution will be higher, so we will have overestimated the available budget.14

However, unlike the IPCC assessment in 2018,14 the budget does now reflect some possible effects from earth system feedbacks, such as permafrost thawing.13

In the discussions on the How Bad is it? page, a carbon budget of 400 GtCO2 from 2020 is assumed with a 2 in 3 (66%) chance of staying within 1.5°C.
Monitoring our emissions, and hence our remaining budget, is important to know how many years we have left before we reach the limit of 1.5°C. However, it is also important to monitor non-CO2 emissions to fully understand our current state.

Footnotes ▼