1.5 degrees – how big risks are we willing to take?

We are being misled when policies and actions claim to comply with the Paris Agreement – ​​the agreement in which most of the world’s countries committed themselves in 2015 to keeping the global average temperature rise below 1.5°C.

The planetary boundary for global warming was crossed in 1988

The planetary boundary for global warming, 1.0°C, was crossed already in 1988, and we left the so-called safe operating space. At that time, we should have stopped emitting CO2 and other greenhouse gasses.

But we kept on, and the emissions and global mean temperature have been rising at an accelerated pace since then. Therefore, we have – during the last 36 years – moved more and more into the unsafe area.

In the unsafe area, we can no longer be sure that the climate system will still be able to sustain its own functionality. It has come out of balance, and at some point (we don’t know when), the system will reach a tipping point when change becomes self-perpetuating, leading to substantial, widespread, frequently abrupt, and often irreversible impact.

The impacts will lead to lack of clean drinking water, food, medicine, and shelter for many people.

As if this was not a very substantial risk, the 1.5°C limit was “invented” in 2015. The rationale was that if we could just keep the global mean temperature increase below 1.5°C, it should be possible to avert the worst impacts of climate change.

The 1.5°C limit – a risky approach

Climate scientists have made numerous calculations of how much CO2 we can emit from a given point in time before we exceed a global average temperature increase of 1.5°C.

This quantity is called our remaining global CO2 budget[1]. This figure is not set in stone, as there are uncertainties associated with using the large complex models that are used. When the size of this budget is to be determined, one therefore works with probabilities.

This means that for a given remaining CO2 budget there is a probability that compliance with the budget will also keep the global mean temperature rise below 1.5°C.

One usually works with probabilities of at least 50%, 66% and 83%.

If science has calculated a remaining CO2 budget with a probability of 83%, this also means that even if you do everything to reduce and do not emit more than there is room for in the CO2 budget, there is still up to 17% risk that the global average temperature rise will still exceed 1.5°C.

How large is the remaining global CO2 budget?

The results of climate science’s overall research are collected at intervals in thousands of pages of long reports under the auspices of the UN Climate Panel (IPCC – Intergovernmental Panel on Climate Change). However, this typically only happens at intervals of 5-10 years.

Fact-based decision-making must take place based on up-to-date and timely information on key indicators for the state of the climate system and for human influence on the global climate system.

In June 2023, updates[2] were released to the remaining CO2 budget indicated in the IPCC’s latest report AR6. The results are summarized in Table 1.

Table 1: Remaining global CO2-budgets as of 1 January 2023.

So, to answer the question of how large the remaining global CO2 budget is, we must first decide which probability we want for the global mean temperature increase being kept below 1.5°C?

Comparing to daily life situations, would you live in a building or cross a bridge if there was a 10 percent chance of it collapsing? Or five percent? Or one percent? Of course not.

Nevertheless, the IPCC and many other studies base their conclusions about the rate of CO2 emissions reductions on a probability of at least 50% of keeping the global mean temperature increase below 1.5°C.

This also means that there are 50% risk of global mean temperature increase exceeding 1.5°C.

The conclusions about necessary reduction rates are thus based on particularly risky scenarios but are nevertheless used by politicians and decision-makers to implement policies and actions.

When our very basis of life is at stake, we must of course operate with as high probability as possible so that we can maintain the basis of life. In relation to the CO2 budget, the highest probability is 83% under IPCC.

With that in mind, the remaining global CO2 budget is 100 Gton CO2 as of 1 January 2023, see illustration in Figure 1.

Figure 1: Own figure made from data in [3] with a remaining global CO2 budget which gives an 83% probability of staying below a global mean temperature rise of 1.5°C.

Considering that more than 40 Gton CO2 is emitted per year, this budget will be used up already in mid-2025, i.e. next year if we continue the current emissions.

Consequently, there is an immense need for immediate and very large reductions in global CO2 emissions. Goals for net zero emissions in 2030, 2040, and beyond make absolutely no sense in the light of the above.


[1] 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. In Press.

[2] P. M. Forster et al.: Indicators of Global Climate Change 2022: annual update

[3] P. M. Forster et al.: Indicators of Global Climate Change 2022: annual update