Friday, May 27, 2022

Radiation Trends

A few weeks ago I wrote about the normal seasonal cycle of infrared radiation that plays such an important role in the climate of Fairbanks (and everywhere else on the planet).  My real goal in looking at the radiation data was to examine the long-term trends: how much has longwave radiation changed in association with the rise in average temperatures?  And are there any notable changes in shortwave (direct solar) radiation?  (Apologies to those who find this an arcane topic, but at least one reader thought the last post was "fascinating"!)

Let's start with shortwave radiation, i.e. sunshine.  Here's a chart of the 12-month running average, both downward (red) and reflected upward (blue), and the net gain at the surface (black).  Recall that we're looking at data from the ERA5 reanalysis: it's a model, not pure observations, but the model is heavily constrained by satellite data and other inputs.

The shortwave annual means tend to jump up and down in summer because that's when solar radiation is by far the greatest; so an unusually sunny or cloudy month in summer can shift the 12-month average quite a bit.

As for trends, there is no significant trend in downward shortwave radiation, but according to ERA5 there is a decreasing trend in the reflected upward shortwave, particularly in the last 15 years or so.  Since 2007, the total upward shortwave radiation has been 7% lower than from 1950-2006.  This is significant, and I'll comment more below.

With incoming sunshine virtually unchanged and a decrease in reflection, the net gain of shortwave energy has increased slightly, but the change is a much smaller fraction of the average gain: only about a 1% increase since 2007 (for example).

How about longwave radiation?  As a reminder, longwave energy fluxes are a lot larger than shortwave, because they occur both day and night.  Longwave radiation is closely linked to average temperature, so it's no surprise to see a clear upward trend in both downward and upward fluxes: see below.  The downward flux is governed by the temperature and humidity of the air, and the upward flux is controlled by the ground temperature.

Notice the big jump in both upward and downward fluxes in 1976, when the Pacific climate regime (PDO phase) suddenly changed and Alaska warmed up dramatically.  Another significant increase in temperature and longwave fluxes has occurred in the past decade.

However, despite highly significant trends in the upward and downward components of longwave radiation, there has been absolutely no trend in the net longwave.  At first glance this is surprising to me: despite a warmer atmosphere aloft, with higher water vapor content and steadily increasing CO2, there has been no net gain in longwave energy at the surface.  This is because surface longwave emissions have increased by just the same amount (due to surface warming).

Let's look at the monthly breakdown of linear trends over the 72-year history of ERA5 data.  Every month has seen an increase in longwave fluxes, with the trends being statistically significant in more than half of the months.  The greatest trend has been seen in December:

The month-to-month variation in trends corresponds to the monthly temperature trends: compare the charts above and below.  For the temperature trends below, I'm showing both ERA5 model data and GHCN station data from Fairbanks, and the agreement is mostly very good.  Curiously, the trends oscillate up and down from one month to the next in the cold season: October, December, February, and April have warmed relatively quickly, but September, November, January, and March have warmed less.

The greatest warming trend has been seen in December, but again there's no change in the net longwave: the upward and downward fluxes vary in lockstep depending on temperature, and the trends are equal.

Interestingly, August stands out as having different longwave trends, with downward longwave having increased much more than upward longwave, and consequently there is actually a statistically significant change in the net longwave for August (the only month for which this is true).  According to ERA5, this has occurred via a significant increase in August cloudiness, so that the surface has failed to warm over the decades (and thus upward emission hasn't increased), while increased temperatures and humidity aloft have produced more downward longwave radiation.

Here are the shortwave monthly trends, with the August change standing out very clearly: August has become considerably more cloudy, at least according to the model.  The loss of direct solar input is larger than the net gain of longwave radiation, so August has seen a net loss of radiative energy, and this is consistent with the relatively small warming trend in that month.

The other month with a big change in the radiation budget over the decades is obviously April.  In this case Fairbanks has seen a statistically significant net gain of energy caused by a decrease in reflected shortwave radiation.  It doesn't take too much thought to figure out why this is: the surface albedo has decreased as long-term warming has shifted snow melt to earlier dates, so that bare trees and bare ground have emerged earlier in recent decades.  This is obviously a positive feedback, as the darker surface means more absorption of shortwave energy and therefore more warming.

The chart below shows the annual variation in the April fluxes: cold Aprils with prolonged snow cover (like 2013) show up with higher upward (reflected) shortwave and lower net shortwave absorption (black line), whereas warm Aprils like 2016 have low reflection and high absorption.

Notice that the April change in surface characteristics also shows up in the longwave trends (reproduced again below for clarity): with the surface warming more than the air aloft, there's been a net loss of longwave energy in April, but this only offsets about a third of the net gain from shortwave absorption.

The April change is easily the most significant shift in the overall radiation budget for Fairbanks, and it accounts for nearly all of the annual net gain in radiative energy at the surface.  In total, the annual surface energy gain (which is positive, as the last post showed) has increased by about 0.4% per decade, and again this is almost entirely because of the large change in April.

I'll stop here and invite comments from interested readers (if there are any).  And for a later follow-up, perhaps I'll look at the spatial distribution of some of these changes across the state.

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