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Thread: Climate Change & Global Warming

  1. #231
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    Have we entered a new phase of climate change?

    A striking spate of weather extremes has triggered studies to find out if it has become more chaotic

    In just over a week, if all goes as planned, a colossal report on the state of the global climate will emerge from the UN’s Intergovernmental Panel on Climate Change.

    This is the sixth analysis of its kind in 31 years and, like the other five, it will be a sweeping scientific assessment of how and why the planet is warming. Yet this report will be different.

    It will arrive as the impact of a shifting climate seems brutally apparent, not just on remote Himalayan glaciers or Arctic sea ice, but right in front of frightened human eyes.

    In the past four weeks alone, wildfires virtually burnt a Canadian village off the map after it shattered the national record with heat of 49.6C. Floodwaters tore through German towns like a tsunami, tossing cars like corks. Terrified Chinese subway passengers stood in chest-high water as nearly a year’s worth of rain fell in three days.

    Much of this was predicted. Scientists have warned for years that a warming climate would lead to more weather extremes. Yet the frequency and severity of these events raise unsettling questions: could we be entering a period of non-linear climate change, where temperatures and extreme events do not increase smoothly as expected but instead come suddenly, more often and perhaps more powerfully? And if we are, how would we know?

    The short answer is that scientists are divided about whether a more dangerous phase of non-linear change has begun. “I don’t think it’s correct to conclude that’s what we’re seeing, though I have seen people arguing this,” says Michael Mann, director of the Earth System Science Center at Pennsylvania State University. “It’s not so much that climate change itself is proceeding faster than expected — the warming is right in line with model predictions from decades ago. Rather, it’s the fact that some of the impacts are greater than scientists predicted.”

    One of the most striking effects came at the end of June as a prolonged heatwave scorched western parts of Canada and the US Pacific Northwest. Records that had stood for decades were smashed by as much as 5C.

    “That’s just sort of staggering,” says Brian Hoskins, chair of the Grantham Institute for Climate Change at Imperial College London. “For many years I’ve said that the projections from the climate models are what we get if we’re lucky, because their behaviour is very smooth. If you take the output from models, then that heatwave should not have happened.”

    Geert Jan van Oldenborgh, a climate researcher at the Dutch national weather service, says the record North American heat has “shaken the confidence of a lot of climate researchers”. “It means that the assumption that we had about how heatwaves react to a gradual increase in global warming may not be correct,” he says.

    Van Oldenborgh co-leads the World Weather Attribution group of scientists who concluded this month that the North American heatwave would have been “virtually impossible” without human-caused climate change. He and colleagues are now planning wider research that will look at whether there is any evidence to suggest the climate is in fact starting to change globally in a non-linear way. Could it be, for instance, that changes in the jet stream or the migration of drought zones are triggering shifts we do not yet understand?

    This work is notable, considering the role of the scientific phenomenon that researchers such as Harvard historian Naomi Oreskes have called ESLD — Erring on the Side of Least Drama. Climate scientists have been relentlessly accused of fear-mongering and alarmism. But as Oreskes and her colleagues wrote in a 2012 paper, “core scientific values of objectivity, rationality and dispassion” have led to conservative projections about the impact of climate change, even in IPCC assessments.

    This has not stopped the study of exceedingly dramatic concepts such as “tipping points” or thresholds that, once crossed, lead to drastic changes such as the loss of the West Antarctic ice sheet or Amazon rainforest. Indeed, a leaked draft of the new IPCC report suggests it may cover such shifts in more detail than past assessments have.

    Despite, or perhaps because, these scenarios are so bleak, some of the scientists focusing on them have begun to offer more hopeful ideas.

    The logic of a tipping point means it could also set off the irreversible advance of electric cars, renewables and other decarbonising measures, says Tim Lenton, director of the Global Systems Institute at the University of Exeter: “That’s the glimmer of hope.”

    In July 2021, however, it still feels distinctly remote.

    Financial Times

  2. #232

    Atlantic's major current is weakening, signalling significant weather changes – study

    Atlantic's major current is weakening, signalling significant weather changes – study

    THE ATLANTIC OCEAN’S major current, which influences weather systems worldwide, may have been losing stability over the course of the last century, a new scientific study said today.

    The Atlantic Meridional Overturning Circulation (AMOC), to which the Gulf Stream belongs, transports warm water masses from the tropics northward at the ocean surface and cold water southward at the ocean bottom. This plays a major role in creating the relatively mild temperatures found in Europe, notably in Ireland.

    Climate models have shown that the AMOC is at its weakest in more than a 1,000 years. However, previously it has not been known whether the weakening is due to a change in circulation or if it is to do with the loss of stability. “The difference is crucial”, says Dr Niklas Boers from the
    Potsdam Institute for Climate Impact Research, “because the loss of dynamical stability would imply that the AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur.”

    As it influences weather systems worldwide, a potential collapse of the current could have severe consequences across the globe. The study, published in the journal Nature Climate Change, found that the weakening is indeed likely to be associated with a loss of stability. The researchers came to this conclusion by analysing the sea-surface temperature and salinity patterns of the Atlantic Ocean. “The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse,” Dr Boers said in a statement.

    A number of factors are likely contributing to the weakening of the current. These factors include freshwater inflow from the melting of the Greenland ice sheet, melting sea-ice, increasing precipitation and river run-off. Freshwater is lighter than saltwater and reduces the tendency of the water to sink from the surface to greater depths. These factors add to the direct effect that the warming of the Atlantic ocean has on its circulation. The researchers note that, while the respective relevance of the different factors has to be further investigated, they’re all linked to human-related climate change.

    After the ice age ended about 14,500 years ago, Ireland experienced warmer climate only a bit colder than today for a few hundred years but then, at 12,900 years ago, our climate suddenly returned to glacial conditions. It’s known as the Nahanagan Stadial in Ireland, after Lough Nahanagan where Turlough Hill pumped storage power station is. During the construction of Turlough Hill they discovered the lake was formed by a glacier, but oddly after the ice age was supposed to have ended. It’s also known as the Younger Dryas.

    I was told by a researcher who studied a lake in Ireland that recorded the event, that it was like flicking a switch. One year warm, the next year the Ice Age again. The cold climate lasted for 800 years, and may have killed the Giant Irish Elk in Ireland. The cold ended around 11,700 years ago.
    Barnosky, A.D., 1986. “Big game” extinction caused by late Pleistocene climatic change: Irish elk (Megaloceros giganteus) in Ireland. Quaternary Research, 25(1), pp.128-135.

    The cause is not known for certain, but most scientists think it was due the collapse of AMOC due to the influx of fresh water from a giant lake that was formed in North America after the ice age, Lake Agassiz. The influx of fresh water blocked the warm Gulf Stream that warmed Europe. The also caused the end of the native American Clovis culture in North America.

    So, the collapse of AMOC likely happened before and when it did we were plunged into a brief 800 year Ice Age. That said, most scientists think a collapse of AMOC this time won’t cause such catastrophic Day After Tomorrow scenario, but it would not be pretty.

    Atlantic's major current is weakening, signalling significant weather changes – study06 VIII 2021.

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