Himalayan Glacier Loss Is Due To Many Factors
In much of today’s environmental journalism reporters are too quick to attribute almost all changes to anthropogenic forcing.
Many news outlets have reported the very interesting finding detailed in Science Advances that ice loss from Himalayan glaciers has doubled in the past 40 years. Using data from modern satellites and from declassified images taken by the US Hexagon series of spy satellites in the 1970s and 80s the researchers find that for 1975 - 2000 the mass loss was 4 billion tonnes per year increasing to 8 billion for 2000 – 2016. Some of these reports have responsibly said that Himalayan glacier ice-loss is due to many factors, others leave it implied that it is all due to man-made climate change.
Warming rates are not uniform around the world. High-altitude regions, such as the Tibetan Plateau (TP) (sometimes known as the world’s “third pole”) are experiencing faster warming rates than other regions at the same latitude, especially in winter and spring, see here and here. Interestingly, the TP’s warming continued during the global warming hiatus period of 1998–2013 that ended with a strong ENSO. The trend of annual mean temperature over the TP reached 0.44°C decade−1 in 1980–2013, which is 2–3 times greater than that for the Northern Hemisphere at the same time.The recent TP warming has had its influence on glacier retreat, snowmelt, and permafrost degradation, see here, here and here.
Changing patterns of monsoon precipitation (see here, here and here) together with the melting of glaciers, poses a significant threat to the freshwater supply to the densely populated regions downstream (see here, here and here)
Poorly Understood Natural Variability
As ever in science, and it's especially true for climate change, things are somewhat more complicated than many reports indicate. A recent paper in the Journal of Climate shows this. Researchers used well-replicated tree-ring width records and constructed 358 years of summer minimum temperature of the whole Tibetan Plateau (TP). This reconstruction matches the recent warming signal seen since the 1980s, and captures 63% of the variance in 1950–2005 instrumental records
This new paper reports that the decadal temperature cycles seen in the North Atlantic, the so-called Atlantic Multidecadal Variability (AMV), has a pronounced affect on decadal and centennial timescales. Atlantic circulation is a major source of natural variability of Northern Hemisphere climate (Delworth et al. 2016), including multidecadal variations in NH temperature (Knight et al. 2006; Tung and Zhou 2013; Zhang et al. 2007). Indeed, according to the researchers between 1963 – 94 it offset greenhouse forcing–induced warming trends for the entire (TP) leading to anomalous cold summer conditions during this epoch.
In fact, the TP is one of the few regions in Eurasia where minimum summer temperatures reflect AMV changes over 1927–94. Since the mid-1990s, some argue that the positive AMV phase has increased summer warming of Eurasia caused by greenhouse gasses (Zhang et al. 2007).
The researchers conclude that while a quantitative attribution of warming trends to AMV versus anthropogenic forcing is outside the scope of the current study, their findings suggest that both the AMV phase and anthropogenic forcing need to be considered when predicting future of glaciers in the region because natural variability can work to increase and decrease temperatures. They suggest that half of the warming seen in the region over the past 350 years is due to natural variability. The AMV emerges as an essential part of the climate system causing long-term temperature variation in the Tibetan Plateau and it is poorly understood.
In much of today’s environmental journalism reporters are too quick to attribute almost all changes to anthropogenic forcing. If a full picture of the changes undergoing Earth are taken into consideration a more interesting picture emerges. As well as the example of the Tibetan glaciers there is the changing extent of Arctic sea-ice. Recent research suggests that only half of its decline has been due to anthropogenic forcing with the rest due to natural variability, with the latest data suggests that over the past decade its rate of loss has decreased compared to what it was 15-20 years ago.
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