Did Climate Change Make The Hawaii Fires Much More Likely To Happen?

Part 2 of 'What Caused The Hawaii Wildfires?'

The BBC and no doubt other ‘expert’ journalists in the MSM, plus actual ‘experts’ think that it did, applying their usual devastatingly simple ‘Alf Garnett’ logic of ‘stands to reason’.

You can’t argue with Alf Garnett. Well, you can actually. You just have to ‘follow the science’, actual science that is, not stuff just jokingly made up on the hoof by ‘scientists’ like Mandy Cohen:

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Two studies published in 2022 looked at drought occurrence in Hawaii to examine possible causes for the 21st century increase in drought severity and frequency on the islands. What those studies found is both enlightening and informative.

The first is here. I’m just going to quote directly from the paper (my bold):

Hawaii’s recent drought is among the most severe on record. Wet-season (November–April) rainfall deficits during 2010–19 rank second lowest among consecutive 10-yr periods since 1900. Various lines of empirical and model evidence indicate a principal natural atmospheric cause for the low rainfall, mostly unrelated to either internal oceanic variability or external forcing. Empirical analysis reveals that traditional factors have favored wetness rather than drought in recent decades, including a cold phase of the Pacific decadal oscillation in sea surface temperatures (SSTs) and a weakened Aleutian low in atmospheric circulation. But correlations of Hawaiian rainfall with patterns of Pacific sea level pressure and SSTs that explained a majority of its variability during the twentieth century collapsed in the twenty-first century. Atmospheric model simulations indicate a forced decadal signal (2010–19 vs 1981–2000) of Aleutian low weakening, consistent with recent observed North Pacific circulation. However, model ensemble means do not generate reduced Hawaiian rainfall, indicating that neither oceanic boundary forcing nor a weakened Aleutian low caused recent low Hawaiian rainfall. Additional atmospheric model experiments explored the role of anthropogenic forcing. These reveal a strong sensitivity of Hawaiian rainfall to details of long-term SST change patterns. Under an assumption that anthropogenic forcing drives zonally uniform SST warming, Hawaiian rainfall declines, with a range of 3%–9% among three models. Under an assumption that anthropogenic forcing also increases the equatorial Pacific zonal SST gradient, Hawaiian rainfall increases 2%–6%. Large spread among ensemble members indicates that no forced signals are detectable.

That’s a bit gobbledy gookish, so let’s delve further into the paper to explore what the authors are really saying. Firstly, observations indicate that Hawaii was nearly as dry in the early 20th century as it has become in the 21st century, so that’s an immediate fly in the ointment for those claiming ‘stands to reason - it was the climate crisis wot dunnit’:

a. Empirical analysis of Hawaiian rainfall drivers

Historical time series based on two different Hawaiian rainfall indices for November–April (Fig. 1) each reveal that the recent decades of dryness have been in stark contrast to an era of abundant precipitation during the 1960s to 1990s. Considerable decadal variability of Hawaiian rainfall throughout the record is evident. Whereas the University of Hawai‘i HRI that begins in 1920 (bottom) indicates that the decades of the twenty-first century were the driest within its record, the longer GPCC HRI (top) indicates that a nearly comparable dry epoch occurred in the first decades of the twentieth century.

But there’s more:

A century-long drying trend (ending in 2019) is also statistically significant based on University of Hawai‘i data. In contrast, owing to the presence of an antecedent dry era in the early twentieth century, a long-term drying trend spanning 120 years (ending in 2020) is not statistically significant in the GPCC data. That the recent period of dryness, although unusual for its persistence, is only modestly drier than during the early twentieth century is noteworthy and affirms the considerable natural multidecadal variability.

There is no statistically significant drying trend in the longest observational record available, but there is considerable evidence of natural multidecadal variability. In other words, no empirical signal of climate change in Hawaii’s rainfall record is detectable. So you can’t blame climate change for ‘supercharging’ the recent drought which has undoubtedly contributed to Hawaii’s wildfires.

But because we’re looking at real science and real data here (as opposed to Mickey Mouse pseudoscience generated for political convenience), the situation turns out to be rather more complex than a simple examination of natural multidecadal variability:

In sum, the above empirically derived view is that Hawaii should have been wet, not dry, in recent decades based on large-scale atmospheric flow patterns, SST conditions, and their pre-2000 historical linkages with island winter rainfall.

Various reanalyses and satellite-derived estimates of the Pacific basin rainfall anomalies during 2010–19 (Fig. 8) give an impression of the recent Hawaiian drought as having been a “local” phenomenon—a further illustration of the island rainfall outwardly failing to conform to large-scale constraints. That the larger-scale rainfall was above normal during 2010–19 over much of the subtropical Pacific from the date line to 150°W does suggest that the teleconnection with various atmosphere–ocean modes of variability did not break down, on a larger spatial scale.

What this says is that basically, during the first two decades of the 21st century, the Hawaiian islands appear to have decoupled from the main influences of multidecadal variability in terms of rainfall variability; hence they have become drier when they should have become wetter. So can climate change explain this recent anomalous departure from expected natural variability? It’s a reasonable question and the authors do their best to provide a reasonable, robust, scientific answer:

Our empirically derived view is that Hawaii should have been wet, not dry, in recent decades. Both the large-scale observed atmospheric flow patterns and SST conditions were found to be favourable for wet conditions, at least when viewed through the lens of twentieth-century correlations.

A curious observed feature of the Hawaiian drought was its isolated character. Satellite and modern reanalysis products compel a characterization of the recent drought as a local phenomenon, with low Hawaiian rainfall embedded within a larger-scale pattern of high rainfall over much of the subtropical Pacific from the date line to 150°W. The empirical indicators thus implied the teleconnection with various atmosphere–ocean modes of variability did not break down, on a larger spatial scale, but perhaps only with indices of island rainfall itself. In this sense, the recent decadal Hawaiian drought stands in striking contrast to the comparably low Hawaiian rainfall decade of 1906–15.

The empirical analysis raised at least as many questions as it answered, and the paper turned to climate model experiments to better understand what, if any, boundary forcings and associated large-scale climate drivers may have led to this prolonged period of dryness in Hawaii. They included several different ensemble AMIP simulations of the last decade (2010–19) in comparison with the late twentieth century, and a set of so-called event attribution simulations used to isolate the effect of long-term anthropogenic change forcing since the early twentieth century. Concerning decadal variability, four different models agree in revealing a forced signal of a weakened Aleutian low in the recent decade relative to the late twentieth century. However, neither of the model ensemble means yield reduced rainfall near and over the Hawaiian Islands. The absence of a signal and the large spread among members of the model simulations—each subjected to identical decadal boundary forcing variations—indicated that the recent decadal manifestation of Hawaiian drought was best reconcilable with internal atmospheric variability. These model results also support an argument that the anomalous circulation pattern in the North Pacific during the recent decade (the weakened Aleutian low) was unlikely responsible for the drought.

Additional model experiments explored how climate change since the early twentieth century may have affected Hawaiian rainfall in the early twenty-first century. Two different assumptions on the externally forced pattern of SST change, each derived from observed trends since 1900, were employed to address the sensitivity of Hawaiian region rainfall to plausible anthropogenic forcing.

Our results indicate that the direction in which climate change may have contributed to the twenty-first-century low wintertime Hawaiian rainfall depends on the assumption on how external radiative forcing acted to change sea surface temperatures, especially over the equatorial Pacific. It is beyond the scope of this study to determine which, if any, of these two patterns of long-term SST change is the more plausible fingerprint of global warming to date. Instead, it is perhaps more constructive to recognize that, for all the model precipitation change signals described above, their magnitudes are considerably less than their simulated internal variability of climate on centennial time scales (see also Quan et al. 2018). Recall that each ensemble member in our atmospheric model experiments experiences identical ocean boundary forcing and atmospheric chemical composition change, and as such the spread in histograms (e.g., Fig. 13) arises purely from internal atmospheric variability. It is perhaps surprising to see that atmospheric fluctuations alone, unconstrained by boundary forcing changes, could yield centennial-scale Hawaiian rainfall changes on the order of 20%. It is thus entirely possible to reconcile the observed severity of the recent Hawaiian decadal rainfall deficits with unforced internal variability alone.

The authors found that the signal of climate change on rainfall varied in sign between the models, but more significantly, it was of relatively small magnitude compared to unforced natural internal variability; therefore there is no detectable fingerprint of climate change on the recent Hawaiian dry period beginning 2000. Oh dear, how sad, never mind.

A second academic paper comes to similar conclusions:

Drought is a prominent feature of Hawaii’s climate.

We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014.

Recent evidence has called into question the stability of teleconnection patterns in the Pacific [72–76], which has important implications for predicting drought. How these relationships between ENSO and Hawaiian rainfall will change with future warming is still unknown. However, some research indicates that the frequency and intensity of El Niño events will increase significantly [77,78], which could lead to an increased frequency of extreme drought in Hawaii. The unprecedented 2007–2014 drought identified here was not clearly driven by ENSO, however, and this has led to more questions about the mechanisms that drive multi-year droughts in Hawaii. [73].

So, if not ENSO, was it climate change wot dunnit?

Novel categories of drought are emerging due to anthropogenic climate change, expanding human water use, and land use change (e.g., “Hotter Drought” [136]; “FlashDrought” [137]; “Human Induced” or “Human Modified Drought” [138]; and “Transformational Ecological Drought” [21]). These new forms of drought are increasingly difficult to anticipate and manage [139]. Whether droughts in Hawaii are beginning to show characteristics reflective of anthropogenic influence is unclear. However, the frequency, intensity, and duration of droughts were all higher in the second half of the study period (Figures 6,7and 9), with the two longest duration and most severe droughts in Hawai‘i occurring since 1998. Although the 2007–2014 drought was unprecedented over the past century (Figures 2and 6), detecting an anthropogenic signal at small spatial scales such as that of the Hawaiian Islands is difficult, and at this time, evidence indicates that rainfall changes in Hawaii are still predominantly driven by large-scale modes of natural variability [8,73].

Take it away Windsor: