Archive for May, 2020

Record Cold Upper Mesospheric Temperatures Suggest Another Good Noctilucent Cloud Viewing Season

Sunday, May 31st, 2020

Noctilucent clouds (NLCs) are often visible in the extreme upper atmosphere (80-85 km altitude) well after sunset in the late spring and summer at high northern latitudes. They form from ice condensing on meteoric dust at extremely cold temperatures (below 150 Kelvin or -190 deg. F). The cold temperatures are due to adiabatic cooling from dynamic uplift combined with little ozone to absorb the sun’s ultraviolet radiation.

Noctilucent clouds on May 31, 2020 (Alan Tough, Scotland)

With current solar minimum conditions reducing solar heating by ultraviolet radiation, and slowly increasing CO2 in the atmosphere causing enhanced infrared cooling to outer space, record low temperature are occurring. This has extended the southernmost viewing opportunity for NLCs from the usual ~60 deg. N latitude to as low as 35N latitude last year, with NLC sightings near Los Angeles and Las Vegas in 2019.

This recent article by Dr. Tony Phillips suggested that 2020 is shaping up to be another good NLC viewing season. An earlier article by Tony included this nice plot of April-May temperatures at 80 N latitude from this year compared to previous years, showing the general cooling trend during the current solar minimum conditions.

Data plot courtesy of Dr. Lynn Harvey, U. of Colorado.

We downloaded some more recent Aura satellite Microwave Limb Sounder data, and plotted it as a function of latitude. Note how fast temperatures dropped in only 6 days… 1-2 deg. C per day depending upon latitude.

Upper mesospheric temperatures at 83 km altitude on two different days in May, 2020 as a function of latitude. Lines are 2nd order polynomial fits to the data, providing average temperatures as a function of latitude.

The 141 K average temperature on May 27 (Day 148) at 80N latitude appears to be a new record low if we compare it to the data in the previous plot by Dr. Harvey.

I will try to keep track of these temperatures as the NLC season progresses in the coming weeks. So far, NLC sightings have been mostly in northern Europe and the UK.

Why the Current Economic Slowdown Won’t Show Up in the Atmospheric CO2 Record

Friday, May 15th, 2020

Summary: Atmospheric levels of carbon dioxide (CO2) continue to increase with no sign of the global economic slowdown in response to the spread of COVID-19. This is because the estimated reductions in CO2 emissions (around -11% globally during 2020) is too small a reduction to be noticed against a background of large natural variability. The reduction in economic activity would have to be 4 times larger than 11% to halt the rise in atmospheric CO2.

Changes in the atmospheric reservoir of CO2 occur when there is an imbalance between surface sources and sinks of CO2. While the global land and ocean areas emit approximately 30 times as much CO2 into the atmosphere as humans produce from burning of fossil fuels, they also absorb about an equal amount of CO2. This is the global carbon cycle, driven mostly by biological activity.

There are variations in the natural carbon cycle, such as during El Nino (more CO2 accumulation in the atmosphere) and La Nina (more CO2 removed from the atmosphere). Greater wildfire activity releases more CO2, while major volcanic eruptions (paradoxically) lead to greater photosynthesis from more diffuse sunlight and extra removal of CO2 from the air. The most dramatic variations are seasonal, as the land-dominated Northern Hemisphere experiences an annual cycle of vegetation growth (CO2 removal) and decay (CO2 release).

The increase in atmospheric CO2 observed since the 1950s is most likely dominated by anthropogenic CO2 emissions, which are twice as large as that needed to explain the observed rise. As I have shown before, a simple CO2 budget model driven by (1) estimates of global yearly anthropogenic CO2 emissions, (2) El Nino and La Nina activity, and (3) a CO2 removal rate that is proportional to how much “extra” CO2 is in the atmosphere compared to a “preferred baseline” CO2 level, yields an excellent fit to yearly CO2 observations at Mauna Loa, Hawaii.

Fig. 1. Yearly Mauna Loa, HI CO2 observations since 1959 (red) versus a simple CO2 budget model (blue).


But those are yearly measurements, and we are now interested in whether the recent global economic slowdown is showing up in the monthly Mauna Loa CO2 data. If we remove the large seasonal variations (driven by the seasonal growth and decay of Northern Hemisphere vegetation), we see no evidence of the economic slowdown through April, 2020.

Fig. 2. Monthly CO2 data since 2015 from Mauna Loa, HI after the average seasonal cycle is statistically removed.

As can be seen in Fig. 2, there are some pretty large month-to-month jumps and dips around the long-term increase (represented by the dotted line). These are probably natural variations due to fluctuations in the average seasonal variations in vegetation growth and decay, wildfire activity, and El Nino and La Nina activity (which are imperfectly removed in the solid blue line in Fig. 2). Variations in economic activity might also be involved in these fluctuations.

The point is that given the large month-to-month variations in natural CO2 sources and sinks seen in Fig. 2, it would be difficult to see a downturn in the anthropogenic source of CO2 unless it was very large (say, over 50%) and prolonged (say over a year or longer).

Instead, the U.S. Energy Information Administration (EIA) estimates that the global economic slowdown this year due to the spread of the novel coronavirus will amount to only about an 11% reduction in global CO2 emissions. This is simply too small of a decrease in CO2 emissions to show up against a background of considerable monthly and yearly natural variability in the atmospheric CO2 budget.

That relatively small 11% reduction also illustrates how dependent humanity is on energy, since the economic disruption is leading to U.S. unemployment rates not seen since the Great Depression of the 1930s. Everything that humans do requires access to abundant and affordable energy, and even the current economic downturn is not enough to substantially reduce global CO2 emissions.

ADDENDUM: How much of a decrease in CO2 emissions would be required to stop the atmospheric rise in CO2?

An interesting aspect of the observed rise of atmospheric CO2 is that it indicates the greater the CO2 concentration, the faster the “extra” CO2 is removed by biological activity. The observed annual rate of removal is 2.3% of the excess above a baseline of 295 ppm. The greater the “excess”, the faster the rate of removal.

Because of this rapid rate of removal, the anthropogenic CO2 emissions do not have to go to zero to stop the observed rise in atmospheric CO2. Using my simple model (blue line in Fig. 1, above), I find that a 43% reduction in anthropogenic CO2 emissions in 2020 would — in the absence of natural fluctuations in the carbon cycle — lead to a halt in the observed rise of atmospheric CO2 in 2020 over 2019 levels. This is about 4 times larger than the EIA estimate of an 11% reduction in CO2 emissions for the year 2020.

UAH Global Temperature Update for April 2020: +0.38 deg. C

Friday, May 1st, 2020

UPDATE: Changed emphasis from Northern Hemisphere extratropics to entire Northern Hemisphere (h/t John Christy)

In April, 2020, the Northern Hemisphere experienced its 2nd largest 2-month drop in temperature in the 497-month satellite record.

The Version 6.0 global average lower tropospheric temperature (LT) anomaly for April, 2020 was +0.38 deg. C, down from the March, 2020 value of +0.48 deg. C.

The Northern Hemisphere temperature anomaly fell from +0.96 deg. C to 0.43 deg. C from February to April, a 0.53 deg. C drop which is the 2nd largest 2-month drop in the 497-month satellite record. The largest 2-month drop was -0.69 deg. C from December 1987 to February 1988.

The linear warming trend since January, 1979 has now increased to +0.14 C/decade (but remains statistically unchanged at +0.12 C/decade over the global-averaged oceans, and +0.18 C/decade over global-averaged land).

Various regional LT departures from the 30-year (1981-2010) average for the last 16 months are:

 2019 01 +0.38 +0.35 +0.41 +0.36 +0.53 -0.15 +1.15
 2019 02 +0.37 +0.47 +0.28 +0.43 -0.02 +1.04 +0.06
 2019 03 +0.35 +0.44 +0.25 +0.41 -0.55 +0.97 +0.59
 2019 04 +0.44 +0.38 +0.51 +0.54 +0.49 +0.92 +0.91
 2019 05 +0.32 +0.29 +0.35 +0.40 -0.61 +0.98 +0.39
 2019 06 +0.47 +0.42 +0.52 +0.64 -0.64 +0.91 +0.35
 2019 07 +0.38 +0.33 +0.44 +0.45 +0.10 +0.33 +0.87
 2019 08 +0.39 +0.38 +0.39 +0.42 +0.17 +0.44 +0.24
 2019 09 +0.62 +0.64 +0.59 +0.60 +1.14 +0.75 +0.57
 2019 10 +0.46 +0.64 +0.28 +0.31 -0.03 +0.99 +0.50
 2019 11 +0.55 +0.56 +0.54 +0.55 +0.21 +0.56 +0.38
 2019 12 +0.56 +0.61 +0.50 +0.58 +0.92 +0.66 +0.94
 2020 01 +0.57 +0.60 +0.53 +0.62 +0.73 +0.12 +0.66
 2020 02 +0.76 +0.96 +0.55 +0.76 +0.38 +0.02 +0.30
 2020 03 +0.48 +0.61 +0.34 +0.63 +1.09 -0.72 +0.17
 2020 04 +0.38 +0.43 +0.34 +0.45 -0.59 +1.03 +0.97

The UAH LT global gridpoint anomaly image for April, 2020 should be available within the next week here.

The global and regional monthly anomalies for the various atmospheric layers we monitor should be available in the next few days at the following locations:

Lower Troposphere:
Lower Stratosphere: