Climate recap - Our World Together

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.

Even small rises in temperature can have big effects, such as the melting of the Arctic ice and the sea level rising. The greater intensity and frequency of many extreme weather events is also attributed to global warming.¹ We have seen reports of intense rainfall, floods, droughts and wildfires, with loss of life, as well as loss of homes and livelihoods. Many of us have experienced these events close by. The page on Impacts provides more detail on the effects of global warming.

Figure 1 shows how much the temperature has increased each year relative to the period 1850-1900. The upward trend begins around 1920 and rises more sharply each year from 1960.

Data sources ▼

Global land and ocean average temperature relative to 1850-1900 average, calculated from HadCRUT5 dataset.

Data source: HadCRUT5 dataset. Met office Hadley Centre: https://www.metoffice.gov.uk/hadobs/hadcrut5/index.html

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

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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

In Figure 2, we can see the concentrations of all 3 gases increasing over the industrial era. Carbon dioxide has the highest concentration in the atmosphere at 415 parts per million (ppm) in 2021. In just a century carbon dioxide concentration has increased by over 100ppm, and it is the first time in over 800,000 years that it has gone above 300ppm.⁸

Data sources ▼

Methane (CH₄) (right axis) and nitrous oxide (left axis) are measured in parts per billion (ppb). Carbon dioxide (CO₂) is measured in parts per million (ppm) (left axis) so is over 1000 times more concentrated than nitrous oxide (N₂0) in the atmosphere.

Data sources:

(Gas concentrations in early years) Ritchie, H., Roser, M., “Atmospheric concentrations” Published online at ourworldIndata.org Retrieved from: ‘https://ourworldindata.org/atmospheric-concentrations‘

(CO₂) Ed Dlugokencky and Pieter Tans, NOAA/GML (gml.noaa.gov/ccgg/trends/). Available here

(CH4): Lan, X., K.W. Thoning, and E.J. Dlugokencky (2022) NOAA/GML: Trends in globally-averaged CH4, N2O, and SF6 determined from NOAA Global Monitoring Laboratory measurements. Version 2022-10, https://doi.org/10.15138/P8XG-AA10 (www.esrl.noaa.gov/gmd/ccgg/trends_ch4/)

(N₂O): Lan, X., K.W. Thoning, and E.J. Dlugokencky (2022) NOAA/GML: Trends in globally-averaged CH4, N2O, and SF6 determined from NOAA Global Monitoring Laboratory measurements. Version 2022-10, https://doi.org/10.15138/P8XG-AA10 (www.esrl.noaa.gov/gmd/ccgg/trends_n2o/)

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How greenhouse gases contribute to global warming ▼

This heat is killing me

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

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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’.

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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).

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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 ▼

IPCC (2018) Table 3.2 Section 3.3.11 Chapter 3. Page 210 in: Hoegh-Guldberg, O, et al: Impacts of 1.5°C Global Warming on Natural and Human Systems. (PDF).

1. IPCC (2018) Table 3.2 Section 3.3.11 Chapter 3. Page 210 in: Hoegh-Guldberg, O., D. Jacob, M. Taylor, M. Bindi, S. Brown, I. Camilloni, A. Diedhiou, R. Djalante, K.L. Ebi, F. Engelbrecht, J. Guiot, Y. Hijioka, S. Mehrotra, A. Payne, S.I. Seneviratne, A. Thomas, R. Warren, and G. Zhou, 2018: Impacts of 1.5°C Global Warming on Natural and Human Systems. (PDF). In: reference 15.

Data source for Figure 1: HadCRUT5 dataset. Met office Hadley Centre: https://www.metoffice.gov.uk/hadobs/hadcrut5/index.html

2. Data source for Figure 1: HadCRUT5 dataset. Met office Hadley Centre: https://www.metoffice.gov.uk/hadobs/hadcrut5/index.html

IPCC (2018) FAQ 1.1 Ch. 1 Page 79 in:Allen, M.R.,et al : Framing and Context. (PDF)

3. IPCC (2018) FAQ 1.1 Ch 1. Page 79. Allen, M.R., O.P. Dube, W. Solecki, F. Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M. Wairiu, and K. Zickfeld, 2018: Framing and Context. (PDF). In: reference 15.

World Resource Institute (WRI) (2018) K. Levin.(2018) Half a Degree and a World Apart: The Difference in Climate Impacts Between 1.5˚C and 2˚C of Warming.World Resource Institute (WRI) Online resource.

4. World Resources Institute (WRI). K. Levin. (2018) Half a Degree and a World Apart: The Difference in Climate Impacts Between 1.5˚C and 2˚C of Warming World Resource Institute (WRI) Online resource.

CarbonBrief (2017) Why scientists think 100% of global warming is due to humans. Hausfather,Z. Online resource.

5. CarbonBrief (2017) Why scientists think 100% of global warming is due to humans. Hausfather,Z. Online resource.

i Further contributions to warming come from increased concentrations of nitrous oxide (N₂0) and the group of gases called the F-gases. The main sources of nitrous oxide are manure from livestock farming, and the misuse of fertilizers in intensive agriculture, whereas industrial processes are the main source of the F-gases.

6. i Further contributions to warming come from increased concentrations of nitrous oxide (N₂0) and the group of gases called the F-gases. The main sources of nitrous oxide are manure from livestock farming, and the misuse of fertilizers in intensive agriculture, whereas industrial processes are the main source of the F-gases.

IPCC (2018) Executive Summary. p 316 in: Chapter 4. de Coninck, H., A. et al: Strengthening and Implementing the Global Response.

7. IPCC (2018) Executive Summary. p 316 in: Chapter 4. de Coninck, H., A. Revi, M. Babiker, P. Bertoldi, M. Buckeridge, A. Cartwright, W. Dong, J. Ford, S. Fuss, J.-C. Hourcade, D. Ley,R. Mechler, P. Newman, A. Revokatova, S. Schultz, L. Steg, and T. Sugiyama, 2018: Strenthening and Implementing the Global Response. In: reference 15.

Hannah Ritchie and Max Roser (2017) – “CO₂ and Greenhouse Gas Emissions“. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions’ [Online Resource].

8. Hannah Ritchie and Max Roser (2017) – “CO₂ and Greenhouse Gas Emissions“. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions’ [Online Resource].

i Note that it is the change in concentrations of greenhouse gases that is causing the rise in temperature. We do need some of the greenhouse gases to trap heat for the planet to be warm, but just not too much.

9. i Note that it is the change in concentrations of greenhouse gases that is causing the rise in temperature. We do need some of the greenhouse gases in the atmosphere to trap heat for the planet to be warm, but just not too much.

i When the 2018 budget was calculated, the remaining allowable warming was calculated roughly as follows: 1.5°C-0.97°C = 0.53°C. That is, the limit on warming of 1.5°C (since the baseline period 1850-1900) minus the estimate of the current warming of 0.97°C (since the baseline), which leaves 0.53°C of allowable warming. This figure is revised downwards by an estimate of the degree of warming still being contributed by non-CO₂ factors at the time of zero net emissions, which gives an allowable remaining warming of 0.38°C (0.53°C-0.15°C). See: IPCC (2018)Section 2.2.2 Chapter 2 Rogelj, J., D. et al. Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development. In: ref 15.

10. i When the 2018 budget was calculated, the remaining allowable warming was calculated roughly as follows: 1.5°C-0.97°C = 0.53°C. That is, the limit on warming of 1.5°C (since the baseline period 1850-1900) minus the estimate of the current warming of 0.97°C (since the baseline), which leaves 0.53°C of allowable warming. This figure is revised downwards by an estimate of the degree of warming still being contributed by non-CO₂ factors at the time of zero net emissions, which gives an allowable remaining warming of 0.38°C (0.63°C-0.15°C). See: IPCC (2018)Section 2.2.2 Chapter 2 Rogelj, J., D. et al. Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development. In: ref 15.

i This is known as the Transient Climate Response to cumulative Emissions (TCRE)

11. i The Transient Climate Response to cumulative Emissions of carbon (TCRE). This is the roughly linear relationship between global mean temperature and cumulative carbon emissions.

i Alternative higher budgets of 500 GtCO and 650 GtCO₂ had a 1 in 2 (50%) and 1 in 3 (33%) chance, respectively, of staying within 1.5°C. IPCC (2021) Table 5.8 Section 5.5.2.3 Chapter 5 Canadell, J.G. et al. Global Carbon and ther Biogeochemical Cycles and Feedbacks. In ref 17.

12. i Alternative higher budgets of 500 GtCO₂ and 650 GtCO₂ had a 1 in 2 (50%) and 1 in 3 (33%) chance, respectively, of staying within 1.5°C. IPCC (2021) Table 5.8 Section 5.5.2.3 Chapter 5 Canadell, J.G. et al. Global Carbon and the Biogeochemical
Cycles and Feedbacks. In ref 17.

i Earth System feedbacks in the context of the climate are processes that increase an initial temperature rise (positive feedback) or decrease it (negative feedback). A negative feedback loop helps to maintain a balance and keep a system within a desirable range of states. A positive feedback loop tends to shift the system out of that range of states. For example, with increased temperatures, sea ice melts. This exposes more of the darker surface under the ice, which reflects less light and absorbs more of the sun’s energy and warmth. This melts even more ice, and so on, in a positive feedback loop. This is called ‘ice-albedo feedback’. For a full explanation of albedo see: NSDC (2020) All about sea ice: Thermodynamics Albedo National snow and ice data center (NSDC) Accessed 10 Oct 2020. Online resource.

13. i Earth System feedbacks in the context of the climate are processes that increase an initial temperature rise (positive feedback) or decrease it (negative feedback). A negative feedback loop helps to maintain a balance and keep a system within a desirable range of states. A positive feedback loop tends to shifts the system out of that range of states. For example, with increased temperatures, sea ice melts. This exposes more of the darker surface under the ice, which reflects less light and absorbs more of the sun’s energy and warmth. This melts even more ice, and so on, in a positive feedback loop. This is called ‘ice-albedo feedback’. For a full explanation of albedo see: NSDC (2020) All about sea ice: Thermodynamics Albedo National snow and ice data center (NSDC) Accessed 10 Oct 2020. Online resource.

i For further discussion of uncertainties in the carbon budget see: IPCC (2021) Section 5.5.2.3 Chapter 5 Canadell, J.G. et al. Global Carbon and the Biogeochemical Cycles and Feedbacks. In ref 17.

14. i For further discussion of uncertainties in the carbon budget see: IPCC (2021) Section 5.5.2.3 Chapter 5 Canadell, J.G. et al. Global Carbon and the Biogeochemical Cycles and Feedbacks. In ref 17.

15. IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. Available from IPCC.

i A sink refers to any reservoir that can absorb more of a gas from the atmosphere than it releases. So forests are carbon sinks because they remove more carbon dioxide (CO₂)than they emit.

16. i A sink refers to any reservoir that can absorb more of a gas from the atmosphere than it releases. So forests are carbon sinks because they remove more carbon dioxide (CO₂₎than they emit.

IPCC 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp. doi:10.1017/9781009157896

17. IPCC. 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on
Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R.
Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.).] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp.
doi:10.1017/9781009157896

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