How much more climate change is inevitable for the UK?

21 April 2020

Kathryn Brown

How much will global efforts to reduce greenhouse gas emissions affect the need to adapt to climate impacts in the UK? Kathryn Brown, the CCC’s Head of Adaptation, takes a look at the evidence. 

One of the key questions that people ask about climate change is “does meeting our Net Zero target mean we can forget about adaptation?” or, to put it another way, “how much more climate change is inevitable?” It’s a good question. We need to understand the answer to it to know just how much climate change we need to prepare for through adaptation. And part of understanding this is working out how much can be avoided through reducing greenhouse gases globally – or mitigation.

The level of climate change we can expect is determined by:

  • The impacts of past emissions that have already changed the climate
  • Global action on mitigation now and into the future and
  • Exactly how strongly greenhouse gases (and other atmospheric components like aerosols) affect global and regional temperature and other climate variables, such as rainfall and sea level rise.

If we look at the latest science from the Intergovernmental Panel on Climate Change (IPCC) and the UK’s Met Office, we can begin to put some bounds around the inevitable amount of future change. We have published these estimates in the table below.

Three messages emerge when we look at the results:

  1. It is likely that the UK will experience at least another half a degree of warming by 2050. Column C in our table shows a central estimate for the minimum temperature increase by 2050. Even with immediate, sustained, and very rapid reductions in greenhouse gas emissions globally, the latest UK climate projections (UKCP18) suggest the country will experience an additional warming of around 0.6°C between now and 2050. This is due to the fact that it will take time for the world to reduce emissions down to Net Zero even under the most optimistic scenarios. Along with this rise in temperature, by the middle of the century we can also expect around another 3 to 37cm of sea level rise for different parts of the UK, a 10% increase in heavy rainfall, and a 50% chance of each summer being hotter than 2018.  These are the minimum levels of changes we must plan to adapt to. Our latest adaptation progress report to Parliament from 2019 suggests that in most sectors, this minimum level of planning is not yet happening.
  2. Reducing global emissions rapidly can still prevent further warming in the UK beyond this ‘inevitable’ level. If global greenhouse gas emissions are brought rapidly to Net Zero in the second half of this century (expected to keep global temperatures below 2°C), UK temperatures (and rainfall) in 2100 (Column D) could be kept close to their level in 2050. However, sea levels in the UK would continue to rise, as they respond more slowly to changes in global temperature. The UK contribution to the global decarbonisation effort is through its target of reaching Net Zero greenhouse gas emissions by 2050 and the intermediate binding carbon budgets. You can read about our latest assessment of efforts to reduce UK emissions here. The message is the same as for adaptation; more needs to be done to meet our domestic mitigation goals.
  3. In the absence of large-scale actions to reduce global emissions, considerably more changes in the UK’s climate will be seen beyond this ‘inevitable’ level. For futures consistent with current global emissions trajectories through to a ‘high-end’ scenario (columns E and F), UK annual mean temperatures would likely increase by a further 2 to 3°C from today by the end of the century. The point at which sea level rise exceeds 1 metre for the UK will also come much sooner; this could even take place within the next 100 years.

What does all this mean?

It means that we have to adapt, whatever happens with UK or global emissions in the future. It also means that the success or otherwise of global efforts to reduce emissions will have a profound impact on the UK’s climate in the second half of this century. And finally, it is clear that mitigation and adaptation together are needed to address the climate challenge. Neither on its own will solve the problem.

 


References:

[i] We use largely use the RCP2.6 scenario in this column that has an approximately 66% chance of keeping warming to below 2°C. Future global emissions scenarios with higher levels of emissions show at least this level of change by 2050. Slightly lower levels of change are seen in the more ambitious scenarios presented in the IPCC Special Report on Global Warming of 1.5°C.

[ii] This column is largely based on the RCP2.6 scenario that has an approximately 66% chance of keeping warming to below 2°C. This scenario sees global emissions fall rapidly to reach net-zero before 2100, with net-removals of CO2 in the final few decades of the century.

[iii] We use the RCP6.0 scenario to approximate this, which is a pathway for future global GHG emissions sees radiative forcing from greenhouse gases and other human activities rise to 6 Wm-2 by 2100. In this scenario emissions continue to steadily rise over the rest of the century.

[iv] This column is largely based on the RCP8.5 scenario, which represents a considerable re-carbonisation of the global economy. Emissions would be required to grow to about three times their current levels by 2100 to achieve a forcing level of 8.5Wm-2 by 2100.

[v] Future projections are taken from the RCP scenarios as simulated by the CMIP5 models. The present level is defined as the 2008-2017 decade, and the range given is the 5th-95th percentile across the CMIP5 models (one ensemble member per model).

[vi] IPCC Special Report on Global Warming of 1.5°C. The figure is for human-induced warming to date and has a +/- 0.2°C uncertainty (>66% chance).

[vii] This scenario achieves net-removal of CO2 from the air before 2100, so global temperature may begin to decline before the end of the century in this scenario, with some probability that temperatures could be returned to a lower level than today by 2100. More ambitious scenarios were assessed by the IPCC Special Report on 1.5°C that have greater rates of CO2 removal and lower ranges of 2100 temperature.

[viii] Future projections are taken from the probabilistic projections from UKCP18. 50th percentile changes are given for the UK annual average temperature, expressed relative to 2008-2017 by accounting for the observed change relative to the 1961 – 1990 reference period (+0.62°C). Inevitable mid-century changes are calculated from the RCP2.6 scenario, for the period 2040 – 2069. The 10th-90th percentile changes given for RCP2.6 over 2040 – 2069 are +0.69°C to +2.24°C from the 1961 – 1990 baseline. For RCP2.6 the 10th-90th percentiles changes for 2070-2099 are +0.72°C to +2.48°C relative to 1961-1990. For RCP6.0 the 10th-90th percentiles changes for 2070-2099 are +1.51 to +4.06°C relative to 1961-1990. For RCP8.5 the 10th-90th percentiles changes for 2070-2099 are +2.21°C to +5.48°C relative to 1961-1990.

[ix] This is an observed change reported in the UKCP18 Science Overview Report. Present levels is defined as 2008 – 2017. See page 11. UK human-induced warming relative to pre-industrial is estimated using the method of Hawkins et al (2020).

[x] Figures from IPCC Special Report on Oceans and the Cyrosphere (2019). RCP2.6 is used for the inevitable change by 2050. RCP6.0 results are only available from the IPCC 5th Assessment report for 2100 and may be slightly inconsistent with the more up-to-date assessment for RCP2.6 and RCP8.5 in the recent special report. Present levels are defined as the average of the 2010-2019 decade with the estimate of the observed change from 1900 (1890-1909 average) is derived from data updated from Church et al (2011). The 2010-2019 decade is 6.6cm higher than over 1986-2005. Projections are expressed relative to the 2010-2019 decade by using this offset.

[xi] Calculated from figure 40 in Kendon et al. (2019) State of the UK Climate 2018.

[xii] Range given for 2060 under the RCP2.6 scenario for different locations (capital cities) around the UK, from a 1981-2000 baseline in UKCP18 Science Overview Report, Pg56.  We have adjusted to be from a baseline of 2010-2019) by subtracting 3.36cm (based on a trend of 0.14 cm/yr) which approximates the difference in sea level between the 1981-2000 baseline and 2010-2019, then rounding to the nearest 1 cm.

[xiii] Heavy rainfall is here defined as the mean of the wettest 5% in the distribution of hourly rainfall over winter.

[xiv] Trends in heavy rainfall are very sensitive to the metric chosen.  See Kendon et al. (2019) State of the UK Climate 2018.

[xv] This is very metric dependent, but a broad estimation is provided in Sayers et al. (2015) Projections of future flood risk for the UK – Appendix C, page 21.Future projections are taken from a scenario expected to reach a peak warming around 2°C for the 2050s and the 2080s, and for a ‘high++’ scenario for the 2080s projections under a high carbon future.

[xvi] UKCP18 Science Overview Report, pg4, 27.

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