Feedback effects: fire & ice
(Image: Michael Held)
Feedback effects: fire & ice
A feedback that increases an initial warming is a ‘positive feedback.’
A feedback that reduces an initial warming is a ‘negative feedback’.
- The main positive feedback in global warming is the increase in the amount of water vapour in the atmosphere, which in turn leads to further warming for reasons explained here.
- Once certain tipping points are reached, the feedback effect becomes self-sustaining. That is, it can’t be reversed.
- Two profound and unprecedented feedback effects now underway are: melting ice and raging wildfires.
Example: Albedo Effect—ice and snow
Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space. The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it (Fig. 1).
As the climate has warmed, the amount of summer sea ice in the Arctic over the last 40 years means has declined (Video 1). The loss is not just in the area covered, but also the thickness. In total, Arctic sea ice has declined 95% in the past 33 years.
This means more heat-absorbing open ocean is exposed (as ice thins, it has a progressively lower albedo), leading to more warming, and so on every year in a downward spiral. This triggers a cascading series of feedbacks that affects weather, changes how ocean currents work, and melts permafrost and methane clathrates, which is releasing more of the greenhouse gasses carbon dioxide and methane into the atmosphere. This in turn leads to even more warming, which in turn melts more ice. The overall impact is that warming in the Arctic is twice the global average (Fig. 2).
Video 1: Loss of sea ice in the Arctic is creating a positive feedback effect, leading to more warming.
As the climate warms, evaporation dries out vegetation, making it more prone to fires. This triggers a range of positive (warming) feedback effects, depending on where the fires are located. This is resulting in the explosive growth of forest forests globally, with profound feedback impact.
To fully explore this global problem, see ‘How climate change is affecting wildfires around the world’ (Carbon Brief).
Wildfires in Australia impact New Zealand
Until 2019, Australia’s national fire-related carbon emissions averaged 439 million tonnes/year. In the first 6 weeks of 2020 alone, fires emitted 830 million tonnes.
The effects were felt in New Zealand when ash and smoke blew across the Tasman (Fig. 3). One afternoon our skies turned orange and for the next few weeks, ash fell over already retreating glaciers, reducing their albedo, leading to faster melting (Fig. 4).
As the climate warms, the weather system in the Indian Ocean, the Indian Dipole (the Pacific ‘sister’ of El Niño/La Niña) is expected see more strong “positive” events similar to the one seen in 2019 that contributed to the Australian drought and bushfires.
Wildfires in the Arctic
“As we move into the 2020 Boreal and Arctic wildfire season in the Northern Hemisphere, parts of the Arctic Circle have been more than ten degrees warmer than usual over the last couple of weeks.” – Copernicus Atmospheric Monitoring Service, May 2020
“Wildfires that incinerated tundra along the Arctic Circle this summer released a record 244 megatonnes of carbon dioxide—35% more than last year, which was also a record breaker.” – Nature, September 2020
Every year, the wildfire season in the Northern Hemisphere (Alaska, Canada, and Russia) begins earlier, ends later, and is more intense. The fires palls of soot or ‘black carbon‘ over Greenland (Fig. 5) reducing the albedo and enhancing surface melting, which in turn speeds up the disintegration of outlet glaciers that hold back the massive Greenland ice sheet. This in turn increases the speed of rising sea levels.
“The entire Canadian Boreal contains 307 billion tonnes of carbon…as much carbon as the world emits in 36 years.” – Anthony Swift, Natural Resources Defense Council, Canada
In addition to losing forest, fires in boreal regions are threatening to turn what had once been a carbon sink, into a major source of atmospheric methane and carbon dioxide. The peat in this region is largely permafrost, but that’s now melting at an alarming rate, both from atmospheric warming and from increasingly uncontrollable forest fires melting the upper layers of the soil (Video 2).
Video 2: As Arctic summers get warmer and drier, boreal forest fires are becoming more intense, meaning they burn deeper into the soil. (NASA)
“Boreal forest fires emit large amounts of carbon into the atmosphere primarily through the combustion of soil organic matter… Climate warming and drying has led to more severe and frequent forest fires, which threaten to shift the carbon balance of the boreal ecosystem from net accumulation to net loss, resulting in a positive climate feedback… This implies a shift to a domain of carbon cycling in which these forests become a net source—instead of a sink—of carbon to the atmosphere over consecutive fires.” – Walker et al, 2019
The Australian Government report states that, “The 2019-20 bushfires will have negligible impact on Australia’s progress towards its 2020 or 2030 target.” (p3) and “...affected forests are expected to recover over time, generating a significant carbon sink in the coming years.” (p9).
Evidence to support this claim is lacking and contradicts scientific concern that entire ecosystems may have been permanently lost (see for example Yale University News). While Australian forest ecosystems have indeed adapted to fire, the 2019/2020 fires were extraordinary, wiping out 186,0002km. That’s an area 30% larger than the entire South Island of New Zealand.
When ecosystems tens millions of years in the making are decimated in just a few weeks, their recovery and replacement in a progressively warmer dryer climate may be vastly different and far less capable of storing carbon. Moreover, the cumulative effect of worsening forest fires each year has been ignored. This industry-led ‘Government’ report should therefore be read in light of the Australian Government’s stance on climate change and ongoing land clearing and coal-mining policies.
Note: the figures in tonnes (above) are taken from https://atmosphere.copernicus.eu/, which use tons (Imperial). These have been converted to tonnes (metric) for consistency.
References and further reading
- National Snow and Ice Data Centre (NSIDC)
- European Centre for Medium-Range Weather Forecasts: Copernicus Atmospheric Monitoring Service
- 2021: Cai et al; Opposite response of strong and moderate positive Indian Ocean Dipole to global warming; Nature Climate Change 11, pp27-32
- Article by authors explaining their findings in Carbon Brief
- 2020: The journal Nature has compiled this open-access (free) series of peer-reviewed paper on wildfires and their impact on ecosystems, contribution to climate change, and damage to human health.
- 2020: Dunne/ Carbon Brief; Explainer: How climate change is affecting wildfires around the world
- 2020: Witze; The Arctic is burning like never before — and that’s bad news for climate change Nature 585, pp336-337
- 2020 Australian Government: Estimating greenhouse gas emissions from bushfires in Australia’s temperate forests: focus on 2019-20, Australian Government Department of Industry, Science, Energy and Resources.
- 2020 Yale News: ‘Wiped out forever’ — the ecological impact of Australia’s wildfires
- NASA: 2019 Arctic Sea Ice Minimum Tied for Second Lowest On Record
- 2019: Thackeray et al. An emergent constraint on future Arctic sea-ice albedo feedback, Nature Climate Change 9, pp972-978
- 2019 IPCC: Special Report on the Ocean and Cryosphere in a Changing Climate
- 2019: Walker et al; Increasing wildfires threaten historic carbon sink of boreal forest soils, Nature 572, pp520-523
- 2019: Aaltonen et al: Forest fires in Canadian permafrost region: the combined effects of fire and permafrost dynamics on soil organic matter quality, Biogeochemistry 143, pp257–274 (open access)
- 2019: The Guardian New Zealand glaciers turn brown from Australian bushfires’ smoke, ash and dust
- 2018: Davy & Trompetter; Black Carbon on New Zealand, GNS Report 2017-122
- 2018: Climate and Clean Air Coalition: Annual Science Update – Black Carbon Briefing Report
- 2018 NOAA Artic Report Card: Executive summary
- 2017: Kashiwase et al; Evidence for ice-ocean albedo feedback in the Arctic Ocean shifting to a seasonal ice zone, Nature – Scientific Reports 7, 8170
- 2005: Dargavel (ed.), Australia and New Zealand Forest Histories, Australian Forest History Society Inc. Occasional Publications, 1