Climate Change – 2020 was this the warmest year?
It’s early days, but 2020 is shaping up to be the warmest year, globally, since records began in the 19th century, and very probably the warmest year Earth has experienced since the end of the last Ice Age, 11,500 years ago. Usually, when a new record is set, it coincides with an above average intensity El Nino, which is a periodic warming of the tropical Pacific. This was the case during the record intensity 2016 El Nino (on a par with the then record 1997-98 event). 2016 is the current global temperature record year. El Ninos usually last for several months, straddling the end of one year, and the start of the following year. The global temperature anomalies for the first four months of the following year are almost always the most extreme, and this was the case in 2016.
According to the NASA GISS data, 2016 was 1.02 C warmer than the average for 1951-80. The Jan-Apr anomaly averaged 1.26 C, but only some of this large anomaly would have been due to El Nino. Something else is going on, which I’ll get to later. For comparison, the weak or non El Nino (ENSO neutral) years of 2017, 2018 and 2019 had global temperature anomalies of 0.92 C, 0.85 C and 0.98 C, respectively. The Jan-Apr average for those three years was 0.98C. The May-Dec average for 2017-19 was 0.89 C, or 0.09 C less than the Jan-Apr average.
Despite no El Nino, the average anomaly for the first four months of 2020 was a whopping 1.19 C. If we assume that May-Dec monthly anomalies average the same 0.09 C less, this would give an annual global temperature anomaly of 1.13 C, obliterating the 2016 record in an ENSO neutral year. This is quite dramatic stuff, and the questions that arise are 1) is this the start of a global warming surge, probably due to positive feedback effects starting to kick in, and 2) if so, what’s causing it?
Most of the warming caused by greenhouse gases (GHGs) is not due to the direct reradiation of outgoing infrared radiation back to the surface. It’s caused by positive feedback effects. For example, water vapour is a much more potent greenhouse gas than carbon dioxide, and the relatively modest amount of direct warming caused by CO2, methane, and other greenhouse gases, causes more water to evaporate from the ocean, amplifying the initial warming. Cloud formation prevents this feedback effect from becoming ‘runaway’ global warming, which is what’s believed to have happened on Venus.
Other positive feedback effects include the reduction of snow and ice cover, decreasing Earth’s albedo, so that more sunlight is absorbed, and the release of CO2 and methane from melting permafrost, and from warmer soils due to increased bacterial activity. The release of methane from methane ice below the sea bed as the sea temperature rises, particularly under the Arctic Ocean, has also received much attention, but this is more conjectural, although there’s no doubt that vast amounts of methane in ice exist there.
The ocean is the ‘sleeping giant’ of climate change. It’s an enormous heat sink, and it’s estimated that 94% of the extra energy trapped by GHGs is absorbed by the ocean, and most of that is transported to the depths. There is enormous potential for various positive feedback effects to kick in as a result of warming in the ocean, which are poorly handled by the computer models, so we don’t hear much about them. It seems very likely that the recent strong anomalies in the early months of the year are due to heat being given off by the Arctic Ocean in winter. At first glance, this seems counter intuitive. Although the September minimum ice cover area in the Arctic Ocean has decreased considerably in recent years, winter cover has not changed much, but what has changed is the depth of the ice. Although satellite pictures show little change in winter area of Arctic Ocean sea ice, what they don’t reveal is how much it has thinned. And, it’s thinned a lot. Typically, it may only be a metre thick, when it was several metres thick in the past.
Syukuro Manabe is a retired Japanese climatologist who spent most of his career working in the US, and was an early pioneer of computer modelling of greenhouse gas effects on climate, particularly in the area of coupled ocean-atmosphere modelling. A few years ago, I watched a Youtube video of him explaining how thinning Arctic Ocean ice in winter could have a profound effect on global climate. Annoyingly, I haven’t been able to find it, since. When the ice is thick, the heat from the ocean can’t get through to warm the air above. But, when the ice thins appreciably, it can permeate through to warm the air above. The water below the ice is about -2 C, while the air above may be -40 C or lower in winter. So, there is enormous potential for a positive feedback effect involving warming of the air above the Arctic Ocean in winter as the sea ice thins. And what goes on in the Arctic doesn’t stay in the Arctic.
This may be responsible for the high monthly global temperature anomalies in the Jan-Apr period in recent years. If it is, then the extremely high 2020 anomalies is cause for concern, as it might be an early indication that this largely unheralded, but potentially large, positive feedback effect may be about to take off. Many ice core studies in recent years have overturned the previous misconception that climate change is always a gradual process. Analysis of Greenland ice core samples have shown that there have been very rapid sustained temperature changes in Greenland of around 10 C in the past, occurring in just a few years.
The ocean surface water mixes with water at depth. If it didn’t, there would be no oxygen, and no life, in the deeper parts of the ocean. This mixing is dependent on salinity and temperature differences. Warmer surface water expands due to the heat, so is less dense than deeper water, and therefore less able to mix. But, the evaporation at the surface makes it more saline, and this makes it more likely to sink. Recent research concludes that, as the surface of the ocean gets warmer, it will become more stratified, and less prone to sinking, so that less heat is transported downwards, thus greatly enhancing surface warming. Presumably, this will mean that the 94% figure for the amount of energy trapped by GHGs that is absorbed by the ocean will drop, and there will be a surge in atmospheric warming as a result. Yet another potentially large feedback effect that you don’t hear much about.
Much of the CO2 produced by man goes into the ocean. This is dependent on the temperature of the ocean surface, and the amount of CO2 already dissolved. The colder regions absorb CO2, while the warmer regions actually outgas it. Therefore, as the oceans warm, there will be less uptake, and more outgassing. In theory, the ocean could eventually become a net emitter of CO2 to the atmosphere, a nightmare scenario. The same thing happens with oxygen. Marine phytoplankton and seaweeds produce oxygen, but it is also absorbed from the air. As with CO2, this is temperature dependent, and some warmer regions may outgas oxygen to the point where they become anoxic ‘dead zones’. Such zones are often seen close to shore, as a result of fertiliser runoff, but anoxic dead zones are increasingly being identified in warmer oceanic regions far from the influence of agriculture.
In the distant past, mass extinction events were associated with high atmospheric CO2 levels, global warming, anoxia in the oceans, and a mysterious terrestrial charcoal layer, indicating that the world burned. It only takes a small increase in atmospheric oxygen to make the world much more combustible, and high levels of atmospheric oxygen, due to outgassing from warmer oceans, and increased lightning due to a warmer climate is a plausible explanation for the charcoal layer.
Wully Davidson, May 2020
This section: Science: Climate Change and Other Topics
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