This second post reporting on the recent Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report Summary for Policymakers looks at what we might expect with different emission levels. The full document is available here to read.
First we should note the five different emission scenarios that the IPCC use:
| Label | Emission Level | What it means |
|---|---|---|
| SSP5-8.5 | Very High | CO2 emissions double by 2050 |
| SSP3-7.0 | High | CO2 emissions double by 2100 |
| SSP2-4.5 | Intermediate | CO2 emissions remain at current level until 2050 |
| SSP1-2.6 | Low | CO2 emissions decline to net zero after 2050 and then negative emissions after that. |
| SSP1-1.9 | Very Low | CO2 emissions decline to net zero around 2050 followed by strong negative emissions after |
SSP stands for Shared Socio-economic Pathway with the first number consistent with previous IPCC reports and the second number is the approximate level of radiative forcing for the scenario in 2100, meaning that the lower the second number, the lower the heating effect. The scenarios differ because of socio-economic assumptions behind them, assumptions regarding levels of climate change mitigation implemented as well as air pollution controls in force. The models have been refined since the 5th Assessment Report resulting in higher sensitivity.
The IPCC report on three timescales:
| Timescale | Years | A child born this year will be |
|---|---|---|
| Near Term | 2021-2040 | In their childhood or teenage years |
| Mid Term | 2041-2060 | In their twenties and thirties |
| Long Term | 2081-2100 | In their sixties and seventies |
Net negative emissions mean when our removal of greenhouse gases from the atmosphere exceed our emission of them.
Future emissions cause future additional warming, with total warming dominated by past and future CO₂ emissions
IPCC Sixth Assessment Report, August 2021
Surface temperatures will increase under every emissions scenario, however only the lowest emissions scenarios with negative emissions have a chance of limiting global warming to 1.5°C and 2°C (relative to measured temperatures from 1850-1900)
| Emission Pathway | Long-term range of temperature rise |
|---|---|
| Very Low | 1.0-1.8 |
| Intermediate | 2.1-3.5 |
| Very High | 3.3-5.7 |
The last time global surface temperatures were greater than 2.5°C from the pre-industrial era was more than 3 million years ago. That’s back in the Pliocene when perhaps Australopithecus started using simple stone tools (and the sea level was 30 metres higher than it is today).
The report includes a graph which is reproduced here to illustrate the predicted change in average surface temperature for each scenario:
Each scenario has similar surface temperatures in the near term before the change in human behaviour is reflected in surface temperature changes in the mid and especially long term.
There may be years where temperatures exceed the trend due to natural variation. The natural level of variability is around ±¼°C in any given year.
Increased global warming means increased frequency and intensity of hot extremes, heatwaves, heavy rain, droughts and tropical storms. It also means reduced Arctic ice, snow cover and permafrost.
The land surface will continue to warm more than the ocean surface. The rate of warming in the Arctic will be above twice the global warming rate. There will be an increased occurrence of unprecedented extreme events.
Extreme daily rain events will increase with the temperature rise. The proportion of category 4 and 5 tropical cyclones will increase with global warming. An ice-free Arctic is likely in a September before 2050.
| Event (1850-1900) | Now (1.0°C) | 1.5°C (Best Scenario) | 2°C (Paris Agreement) | 4°C |
|---|---|---|---|---|
| 1 in 10 year “hot” year | Now 2.8x more likely | 4.1x more likely | 5.6x more likely | 9.4x more likely |
| 1 in 10 year “hot” year | Now +1.2°C hotter | +1.9°C hotter | +2.6°C hotter | +5.1°C hotter |
| 1 in 50 year “hot” year | Now 4.8x more likely | 8.6x more likely | 13.9x more likely | 39.2x more likely |
| 1 in 10 year “intense rainfall in a day” | Now 1.3x more rain | 1.5x more rain | 1.7x more rain | 2.7x more rain |
| 1 in 10 year “dry” year | Now 1.7x more likely | 2.0x more likely | 2.4x more likely | 4.1x more likely |
“With every increment of global warming, changes get larger in regional mean temperature, precipitation and soil moisture”
IPCC Sixth Assessment Report, August 2021
Further warming is expected to intensify the global water cycle resulting in
- more variability
- increased global monsoon precipitation
- increased severity of wet events
- increased severity of dry events
Expect to see more rainfall at higher latitudes, in the mid pacific and in some monsoon regions. Expect to see less rainfall in parts of the subtropics and a few areas of the tropics.
Projected changes in extremes are larger in frequency and intensity with every additional increment of global warming
IPCC Sixth Assessment Report, August 2021
Scenarios with increasing CO2 emissions have carbon sinks (land and ocean) that are less able to sequester the CO2 so the proportion of CO2 taken out of the atmosphere reduces. A higher amount of the emitted CO2 will stay in the atmosphere.
Additional risks from carbon dioxide and methane emissions coming from natural sources as a result of a warmed planet are not fully accounted for.
| Scenario | CO2 emissions taken by land and oceans |
|---|---|
| Historic (1850-1900) | 59% |
| Very Low (SSP1-1.9) | 70% |
| Low (SSP1-2.6) | 65% |
| Intermediate (SSP2-4.5) | 54% |
| High (SSP3-7.0) | 44% |
| Very High(SSP5-8.5) | 38% |
“The proportion of CO₂ emissions taken up by land and ocean carbon sinks is smaller in scenarios with higher cumulative CO₂ emissions”
IPCC Sixth Assessment Report, August 2021
Some of the changes underway are irreversible for very long timescales. Ocean warming and acidification are likely to continue, as are loss of glaciers, ice sheets and permafrost. The sea level will also increase for the rest of the century. Very low emission scenario would limit the sea level rise to 30 cm to 50 cm (12 to 20 inches), but very high emissions would risk up to 1 metre (3 feet) by the end of the century. There is the risk that by 2150 under a very high emissions scenario the sea level rise could by as high as 5 metres (16 feet). Sea level rise will continue for the next millennium. If we keep to 1.5°C increase (our most optimistic target) the sea level rise will be limited to around 3 m (10 ft) over the next 2000 years. Keeping to the Paris Agreement (2°C) would limit sea level rise to 6 m (20 ft), whereas a 5°C of warming would risk up to 22 m (72 ft) of sea level rise over the next 2000 years.
What cities and buildings that you like and enjoy today will be like the lost city of Atlantis in 2000 years? And how much of our cultural antiquity will be lost too?
“Human activities affect all the major climate system components, with some responding over decades and others over centuries”
IPCC Sixth Assessment Report, August 2021
Read the next post on Risk and Adaptation to learn about climate-related risks and adaptation planning.
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