Naturbrande/størrelse /hvor/returperioder

Jørgen Petersen skrev:
At omfanget af skovbrande i 1930'erne var langt end nu er da et rigtigt godt bevis.

(A) Annual frequency of large (>400 ha) western U.S. forest wildfires (bars) and mean March through August temperature for the western United States (line) (26, 30). Spearman's rank correlation between the two series is 0.76 (P < 0.001). Wilcoxon test for change in mean large–forest fire frequency after 1987 was significant (W = 42; P < 0.001). (

First principle component of center timing of stream-flow in snowmelt dominated streams (line). Low (pink shading), middle (no shading), and high (light blue shading) tercile values indicate early, mid-, and late timing of spring snowmelt, respectively. (C) Annual time between first and last large-fire ignition, and last large-fire control.

Hvis man vil påvise nogetsomhelst om skovbrande, så skal der medtages alle forhold

Årsagen er her ret simpel,

Those sceptical about the role of climate change in the recent increase in fires have pointed to the full dataset, trying to argue that the fire area has decreased by around 80% over the past century.

This is not an accurate comparison, according to Randy Eardley, a spokesman at the NIFC. As he tells Carbon Brief:

I wouldn't put any stock in those numbers. To try and compare any of the more modern data to that earlier data is not accurate or appropriate, because we didn't have a good way to measure [earlier data]. Back then we didn't have a reliable reporting system; for all I know those came from a variety of different sources that often double-counted figures. When you look at some of those years that add up to 60 or 70 million acres burned a lot of those acres have to be double counted two or three times. We didn't have a system to estimate area burned until 1960, but it was really refined in 1983.

If 50m acres had actually burned in the early 20th century, it would amount to an area of land equal to the entire state of Nebraska going up in flames every year.

Eardley suggests that earlier records were inflated by including areas where fires were purposefully set to clear forests for agriculture, or where rangelands were torched to get rid of sagebrush to improve grazing conditions. Other federal reports suggest that most of the area burned between 1930 and 1950 was in southeastern US and were primarily intentionally set fires for clearing land.

While the early 20th century data is not reliable and likely double or even triple-counted actual fires, Eardley says that it is possible that fire extents were higher back then for a simple reason: there was no large-scale firefighting organisation in the first half of the 20th century. Therefore, fires would burn through larger areas before being extinguished or burning themselves out, particularly when they were not close to towns or settlements.

kfl skrev:
Jeg har ikke haft tid til at sætte mig ind baggrunden for de omfattede naturbrande i Californien, Australien og det sydlige Europa.

Udbredelse historisk ? returperioder etc ? Hvor ?

Jørgen Petersen skrev:
Hvis det var varmen og tørken, som forårsagende skov- og naturbrandene, så skulle de fleste brande være i det sydlige Californien og i nabostarterne Arizona og Nevada, hvor det er væsentligt mere tørt og varmt end i det nordlige Californien.
Brandene hærger derimod i det nordlige og i det centrale Californien samt i starterne i mod nord dvs. Oregon og Washington.

Som tidliger nævnt, så forårsager GW ikke at det bliver mere tørt og brandfarligt alt i alt. GW forårsager derimod, at klimazonerne forskydes en smugle ud i mod polerne. Derfor vil GW (i den udstrækning at den opstår) betyde, at de tørre og brandfarlige områder flytter langsomt ud imod polerne.

Summary. New scientific publications reviewed since January 2020 strengthen the evidence that climate change increases the frequency and/or severity of fire weather – periods with a high fire risk due to a combination of high temperatures, low humidity, low rainfall and often high winds – in many regions around the world. The western United States is among the regions where the trends in fire weather have been most pronounced in the past at least 40 years. Fire activity is influenced by a range of other factors including land management practices. However, land management alone cannot explain recent increases in wildfire extent and intensity in the western US or southeast Australia because increased fire weather amplifies fire risk where fuels remain available.

The new analysis shows that:
Well over 100 studies published since 2013* show strong consensus that climate change promotes the weather conditions on which wildfires depend, enhancing their likelihood.
Natural variability is superimposed on the increasingly warm and dry background conditions resulting from climate change, leading to more extreme fires and more extreme fire seasons.
Land management can ameliorate or compound climate-driven changes in wildfire risk, either through fuel reductions or fuel accumulation as unintended byproduct of fire suppression. Fire suppression efforts are made more difficult by climate change.
There is an unequivocal and pervasive role of climate change in increasing the intensity and length in which fire weather occurs; land management is likely to have contributed too, but does not alone account for recent increases in wildfire extent and severity in the western US and in southeast Australia.