2020’s Cases for Worst Year Ever Heats Up
From the Oceans to the Arctic, 2020 was the hottest year on record
Making the Case
Last year was certainly one for the record books. California suffered its worst wildfire year on record by a landslide, with 4 million acres burned doubling the previous record set a mere two years ago. The state also suffered its largest fire on record, a so-called gigafire, that burned over a million acres. Colorado also had its worst fire season on record, with it’s two largest wildfires ever. Meanwhile, the Atlantic Hurricane season was the worst by a variety of metrics: most named storms in a year, earliest 5th, 6th, …th, 31st storm formation, tied for most tropical cyclones undergoing Rapid Intensification, and the most landfalling Tropical Cyclones in US history with 12. That these climate phenomena happened amidst the worst pandemic and economic crisis in a century strongly suggests that 2020 was, in fact, a bad year. I shan’t belabor discovering and listing all of the records 2020 has broken; 2020 has sufficiently broken this writer to dissuade him from embarking on such an endeavor.
The record-breaking extreme weather of 2020 are not just a statistical oddity. Gigafires and rapidly intensifying hurricanes occurring amidst a pandemic aren’t just some unfortunate coincidence of temporal misfortune, to be chalked up to bad luck or random chance. They are predictable outcomes of our ongoing experiment of burning fossil fuels for energy, and in so doing heating the planet and causing a global energy imbalance that creates climate change. Indeed, climate scientists have long predicted that climate change would lead to worsening wildfires and more dangerous hurricane seasons. That fire and hurricane records are being shattered this early, so often, and by so much is the source of no small alarm for many climate scientists and activists. The planet’s temperature has been steadily climbing for decades, and 2020 continued the heating trend.
The Hottest Year on Record
Every January, a handful of government agencies and scientific groups tally up all the temperature data from around the globe (literally “around”, some the data is from satellites orbiting the planet) for the previous year and calculate a global average temperature. Per their results, 2020 is either the first or second hottest year ever recorded. Although the methods for accounting for data sparse areas vary from group to group, the groups’ results are remarkably similar. As you can see in the graph above, the amount of variability within any given year is low, and the different agencies and organizations have remarkable consistency in more recent years. This is because there is more data, and the data has better spatial coverage (due to the satellite record), so there are less ‘blanks’ to fill in.
The above graph has a 1981–2010 baseline, which is important to keep in mind when observing that the most recent data points. For example, 2020, is ~0.6°C above that 1951–1980 average. However, that doesn’t equal how much global heating has occurred since pre-industrial times. Per Copernicus, the European Union’s climate monitoring agency, 2020 was “around 1.25°C above the 1850–1900 pre-industrial period”. The record heat that ensconced the world in 2020 is the hottest on record *despite* non greenhouse gas factors that have historically reduced global average temperature. The second half of 2020 featured a La Niña event, a climatological phenomenon where the Eastern Equatorial Pacific Ocean is much cooler than average. La Niña’s usually yield lower than average global temperatures to the tune of a few tenths of a degree. Similarly, last year was the solar minimum of the 11-year solar cycle, which typically produces cooler than average temperatures. That the world saw its hottest year on record in the midst of a La Niña and a solar minimum is indicative of just how much climate change has heated the planet.
A common theme among climate deniers is that because cold weather, Winter, and ice still exist, climate change isn’t really happening. This mindset was famously given a spotlight when Republican Senator James Inhofe of Oklahoma brought a snowball to congress as ‘evidence’ that climate change wasn’t real. Inhofe’s theater of the absurd denialism ludicrously presents climate change as a binary: either all of the ice melts and it never snows again, or climate change isn’t real. If it’s not obvious, just because the planet is warming doesn’t mean we no longer have seasons or cold days.
However, it does mean that the *annual* averages will be hotter around the world. A predictable consequence of our steady heating of the planet is visualized in the above map. It shows which parts of the world had either a top 5 warmest or top 5 coldest year on record. If climate change weren’t real, you’d expect about as much red as blue on the map. Instead, a shocking 10% of the planet had it’s warmest year ever last year, including the majority of Russia, much of South America, and a huge swath of the Western Pacific Ocean. In contrast, 0% of the planet had a year that ranked in the bottom 5 years all time in for annual average temperature. This shifting frequency of more record warm to record hot days is exactly what you’d expect to find a warming world, and 2020 bore that expectation out.
2020’s record heat is alarmingly close to the Paris Climate Agreement target of 1.5°C, to which all countries pledged to continue “pursuing efforts to limit the temperature increase to”. Happily, Joe Biden plans to begin rejoin the Paris Agreement starting today, overturning Trump’s efforts to withdraw from the Paris Accord. We still have a lot of work to do in cutting emissions to zero in the near future to limit the global temperature rise as much as possible.
Country Level Temperature anomalies in 2020
There are multiple ways to frame and analyze just how ridiculously hot 2020 was. Suppose you were trying to guess how many of the world’s ~200 countries were hotter than the 1951–1980 average. If there was no climate change and you assume no ties, and global average temperatures were fairly consistent, a reasonable guess would be about half, or 100 of them. The other half you’d expect to be colder than average. Of course, climate change is quite real, and as noted, 2020 was 0.6°C above the 1951–1980 baseline. So how many were above that average?
If you guessed all of them, you’d be correct. There are no countries or regions of the world that were cooler than average in 2020. Countries like Russia and some Eastern Baltic Nations are already a more than a shocking 3°C above this recent baseline. Which is an important reminder that global average temperature increase is precisely what the moniker suggests: a global average. Some parts of the world will face higher annual average temperatures than the global average. Climate change causes more warming over land than the oceans, and more warming in the Northern Hemisphere than the Southern Hemisphere. In the image below, you can see that global land temperatures are already nearly 2°C above the preindustrial (1850–1900) average. Scientists have claimed that 2°C warming was the ‘safe’ limit beyond which we should not push the Earth’s temperature. That 2020 saw global land temperatures nearly hit this temperatures mark is not an encouraging development.
Regional temperature deviations from global average impacted a series of major catastrophes the last few years: wildfires in Australia, California and Siberia were all worsened by record breaking heat and drought, long predicted consequences of a hotter world. The fact that every country in the world was hotter than the average from half a century ago is an indicator of just how dramatically the planet has warmed, an indictment of how poorly we’ve done mitigating climate change as a global society thus far, and suggests how much more work needs to be done to stop climate change from getting worse.
Global Ocean Heat Content Hits an All-Time High
Although the earlier graph of land and ocean temperatures shows that global land temperatures are warming faster than the oceans, that doesn’t mean Earth’s oceans aren’t taking up a lot of the excess heat produced by climate change. A paper by Cheng et. al released earlier this week showed that ocean heat content (OHC) hit an all time high in 2020. The historical record for global ocean temperatures is much shorter than the record for global surface temperatures, and less robust in terms of spatial and depth coverage. However, Cheng and co-authors account for lower data coverage using an arsenal of new and old technology to measure ocean temperatures at varying depths: drifting buoys, moored buoys, gliders (autonomous underwater vehicles), and autonomous pinniped bathythermographs (monitors attached to seals!), to name a few. The data coverage from all of these instruments affords scientists the ability to approximate global ocean heat content.
The data from Lijing Cheng and Michael Mann’s newest paper confirms that we aren’t just heating the surface of the Earth, but much of our global oceans as well. Indeed, it’s estimated that 90% of the heat trapped by greenhouse gases has been absorbed by the world’s oceans, mercifully for all of us surface dwellers. Much of that heat gets trapped near the surface of the ocean, from zero to 500 meters deep. However, one of the more remarkable findings of the new Cheng et. al paper is that the heat from climate change has matriculated into deeper layers of the ocean, with layers as far down as 2000 meters being warmer than the historical average. As you can see below, the entire water column is significantly hotter than the recent past, and the extra warmth from climate change has reached into deeper layers of the ocean.
Much as is the case on land, the spatial distribution of the warming is quite consistent. For all of the regions Cheng and co-authors highlighted, there was a detectable warming signal, at about ~1 Joule/meter2 in the North Indian, Tropical North Atlantic, the Mediterranean Sea, North Atlantic, and Southern Ocean. Climate change is heating some of the most remote regions of our planet, from the ocean wide, to the ocean deep, from the mountaintops, to the North Pole. There is no part of the planet left untouched by climate change, and the 2020 temperature data from around the world indicates climate change is getting worse.
Shocking Arctic Heat
One of the most striking phenomena of 2020’s record-breaking year has been extreme Arctic heat. Much of the Arctic ocean was over 3°C above the historical average in 2020. Over the last 3 months, much of the Arctic was a whopping 5°C above the 1951–1980 baseline. Melting sea ice in the Arctic Ocean is both a consequence of and contributor to the record warmth. As the planet warms, exposed snow and ice melt. The edges of the Arctic ice sheet are some of the first to go, uncovering the dark Arctic Ocean water to the suns’ rays. Whereas thick sea ice has an albedo of .9 (reflects ~90% of solar radiation), open ocean has an albedo of about .1, instead absorbing 90% of the solar radiation. Newly exposed Arctic ocean waters are therefore significantly warmer than the historical average, and further contribute to sea ice melt. It should come as no surprise that Arctic sea ice was near a record low in 2020.
But didn’t CO2 emissions decrease in 2020?
Covid-19 impacted the world in a variety of ways beyond the obvious impacts of a pandemic: it stranded loved ones across oceans, brought global travel to a screeching halt, and caused a massive global economic crisis and massive job loss. One of the ripple effects of the pandemic caused global economic depression was a reduction in global CO2 emissions. According to the Global Carbon Project, 2020’s CO2 emissions will have declined by approximately 7% relative to 2019 emissions. The 7% decline is one of the few ‘good’ news items of 2020, as we need approximately 7% drops in CO2 emissions *every year* for the next 10 years in order to have a chance to limit warming to 1.5°C. Ideally, this won’t require further economic depressions each of the next ten years resulting in decreased energy — and therefore fossil fuel — demand. Instead, a massive buildout of renewable energy infrastructure, technology, and vehicles to replace phased out fossil fuel-based infrastructure could yield massive job growth, reduce global air pollution deaths, maintain or increase global energy supply, reduce climate change risk, and do it all *without* recurring economic crises.
An often-misunderstood point about climate change is that it’s not carbon dioxide emissions (and other forcings, to smaller extents) that are the key metric for how much warming we experience. Rather, the most important factor is the atmospheric concentration of CO2 that is the most responsible for the observed global increase in temperature. While it’s true that carbon dioxide emissions declined in 2020, carbon dioxide concentrations in the atmosphere hit an all time high, at approximately 418 parts per million at the annual peak in May. This atmospheric forcing is the primary contributor to the record heat observed in 2020, with smaller forcings from rising methane concentrations and reductions in atmospheric aerosol concentrations (aerosols reflect sunlight back to space and are also emitted when we burn fossil fuels, thereby masking some of the warming effect of carbon dioxide). Every year that we emit more carbon dioxide, a large percentage of it remains in the atmosphere, increasing the amount of radiation that greenhouse gases trap in the atmosphere.
Like clockwork, atmospheric concentrations have reached record highs every year for decades, and are famously documented in the Keeling Curve (below). While some of the fossil CO2 we emit is absorbed by trees and oceans every year (hence atmospheric concentrations peaking in May annually before trees ‘leaf’ in the Northern Hemisphere), a large chunk remains in the atmosphere. This is why it’s imperative that global CO2 emissions drop to zero; if we stop emitting fossil carbon dioxide, atmospheric concentrations of CO2 stabilize, and so does the climate forcing that’s heating the Earth.
A Note on Interpreting Global Temperature Data & Debunking Denialists
Over the past four decades, there have been many bad faith efforts by fossil fuel companies, conservative think tanks, Republican politicians, and other individuals and organizations to imply either that global warming has stopped, or even that the planet is actually cooling. The Met office’s graph of global average temperatures over the past 220 years shows a clear upward trend in temperatures. In the 1800’s, temperatures were about -0.6°C cooler than the 1951–1980 average, 2020 was about +0.6°C warmer than the same 1951–1980 baseline. Climate deniers and down-players will often point to smaller segments of the above graph to claim there’s “A pause” or more dubiously, that the planet is actually cooling. Remarkably, you can even find these claims on popular sites like Forbes, and dubiously titled blogs from Harvard grad students.
In the image I edited above, I drew approximations of “lines of best fit” for the whole data set in Red and subsets of years, (say, 2016–2020, or 1998–2014) in Light Blue. The Red Line is the important one, it approximates the long-term trend using all the temperature data we have at our disposal, from 1850 to the present.
Conversely, the Light Blue Lines are approximations of temperature trends for segments of the data. The reader will note that all of the Light Blue Lines I’ve drawn show decreasing trends that are at odds with the long-term trends, a common tactic of climate deniers. This is because if you only look at small subsets of the total dataset, you can *arbitrarily* pick a local high point, say the El Niño years of 1998 or 2016 (El Niño years involve a lot of heat release from the Eastern equatorial Pacific ocean to the atmosphere, and are hotter than neutral years and La Niña years on average), and then claim that Temperatures are decreasing and the planet is actually cooling! These bad faith arguments and bad faith uses of data ignore natural variability in the Earth’s climate system.
Because there’s natural variability in the global climate system, any given year might be modestly warmer or colder than the year before or after it. This is similar to how any given Summer day in your hometown might be slightly hotter or colder than the days before or after. It’s natural variability. Imagine an unusually hot day in early Spring that causes you to get your shorts out or make a break for the beach, followed by a series of average temperature or below average temperature days that have you reaching for your Winter coat. Suddenly, a Summer denier claims that Summer isn’t actually coming this year, and your hometown is actually going to into a second Winter! This absurd claim would presumably be ignored, but this is effectively the same argument that climate deniers make with the blue lines in the image above. They take snippets of data that fit their claim “climate change isn’t real/happening” and then ignore the data/evidence that doesn’t fit the claim, specifically the full temperature data-set going back to 1850.
To reiterate, there is a clear long-term trend of warming. One of the reasons we can have confidence in these trends, is that after certain years, ~1940, ~1983, ~1998, and 2016, every subsequent year is *ENTIRELY* outside the range of previous natural variability in global average temperature. This is demonstrated by the straight Orange Lines I’ve added to the chart. For example, each year since 2016 is completely outside the variability of the other 166 years in the dataset. Similarly, every year since 2001 is warmer than all but one year since before 2001 (the super-El Niño of 1998), the 2001–2015 stretch is outside the variability of the previous 152 years. That 2016–2020 is outside the variability of the 2001–2015 stretch points to just how rapidly the planet is warming right now.
A Longer perspective
Lest you think the temperature records of the last 170 years are themselves a biased sample of a larger temperature timeline, winding back the clock a few thousand years is a revealing exercise. A look further back into pre-recorded global average temperature via tree rings, ice cores and sediment samples sheds more light on the current, human-induced warming. The estimated 1.2°C warming over the past 100 years is unparalleled in the previous 2000, as the graph above from Ed Markey illustrates. Michael Mann of Penn State famously suggested a different version of the above graph looked like a hockey stick. Whatever the graph looks like, the trend is pretty clear: our use of fossil fuels is causing the planet’s temperature to increase drastically at a much more rapid rate than at any point in the last couple millennia. 2020’s record breaking heat is just the latest data point in a centuries long trend of warming. It’s only the worst year ever in terms of temperature for now, so long as we continue burning fossil fuels, 2020’s reign as the hottest year on record will be very short-lived.
