"What happened to greenhouse warming during mid-century cooling?"
And could global brightening be causing global warming?
A few weeks back while researching global brightening, I came across a gem of a paper: Impact of Global brightening and dimming on global warming (Wild et al 2007). The paper examines temperature trends over the second half of the 20th Century, including the cooling period in the middle of the century. From the 1950s to early 1980s, while CO2 levels were rising, global temperatures cooled slightly. How can this be if CO2 causes warming? Wild 2007 found there was greenhouse warming during this cooling period and they find it in an interesting place…
The paper looks at trends in the amount of sunlight reaching the ground over the latter 20th century. Various factors can affect how much sunlight gets through to the Earth’s surface, with the amount of aerosols in the atmosphere being the main contributor. And of course, the amount of sunlight reaching the surface will have an effect on global temperatures. Wild 2007 attempts to disentangle just how much contribution this surface dimming and brightening has on global temperature.
They start by looking at measurements of surface radiation from 1958 (when widespread measurements began). They find a period of “global dimming” from 1958 to 1990 where surface radiation fell. Afterwards, the dimming levels off and transitions to slight brightening from 1985 to 2002. While the warming during the period of solar dimming is moderate, the warming is more rapid in the last two decades where dimming was no more present.
Temperature change over global land surfaces from 1958 to 2002.
How much does global dimming and brightening contribute to the temperature trends. To disentangle the effects of dimming and brightening from greenhouse warming, Wild digs a little deeper into the temperature record by looking at the daily temperature cycle. Sunlight affects the daily maximum temperature more than the nightly minimum, which is affected more by the greenhouse effect. What they find is from 1958 to 1985, during global dimming, the maximum daytime temperature falls. This makes sense as less sunlight is reaching and warming the Earth’s surface. The interesting result is that over this period, the nighttime minimum temperature increases. While global dimming was cooling temperatures in the daytime, the increased greenhouse effect was warming in the nighttime.
From 1985 to 2002, the warming trend during the daytime increases significantly and almost catches up to the nighttime warming trend (almost but not quite). This is consistent with the surface radiation measurements which find global dimming levels off or transitions to brightening in the mid 1980s. Global dimming masked greenhouse warming until the 1980s. Once the atmosphere cleared and the dimming was removed, global warming came into its own.
Does this mean global brightening could be responsible for global warming? Not quite. While there has been some global brightening since 1985, the amount of sunlight hitting the Earth hasn’t reached 1960 levels yet. Sunlight has fallen since 1960 while global temperatures have risen 0.8°C. The daily temperature cycle indicates it’s greenhouse warming that has driven the warming. Yet another human fingerprint!
Where did CO2 warming go during mid-century cooling? Global dimming caused by pollution masked the increased greenhouse effect. Nevertheless, the CO2 warming was still percolating away while we were sleeping.
JR: Here’s the global brightening post.
One skeptic argument, employed by Christopher Monckton in his testimony to US Congress, is that global brightening is the cause of global warming. From 1983 to 2001, the amount of sunlight hitting the Earth’s surface has increased by 1.9 W/m2. Monckton compares this to the radiative forcing from manmade influence since pre-industrial times, estimated at 1.6 W/m2 (IPCC AR4). Monckton argues that these numbers prove global brightening is responsible for recent global warming. But is this the full picture?
Monckton’s numbers come from Do Satellites Detect Trends in Surface Solar Radiation? (Pinker et al 2005). This study analyses satellite measurements of solar radiation, upward radiation from the Earth and cloud cover fraction to model the amount of sunlight reaching the Earth’s surface. They calculate an overall increase in surface solar radiation of 0.16 W/m2 per year. Once the satellite data is corrected to remove an orbital decay bias (ERBE 2005), Monckton calculates a net increase in surface radiative flux of 1.9 W/m2.
Figure 1: Changes in solar radiation at the Earth’s surface from 1983 to 2001. Solid line is linear fit, dotted line is quadratic fit. The linear slope (solid line) is positive at 0.16 W/m2 per year (Pinker et al 2005).
Is it valid to compare changes in surface solar radiation to radiative forcing? A good person to answer this is Rachel Pinker herself who in responding to Monckton’s argument, said the following:
‘The CO2 ‘radiative forcing’ value that Mr. Christopher Monckton is quoting refers to the impact on the Earth’s Radiative balance as described above. The numbers that we quote in our paper represent the change in surface SW due to changes in the atmosphere (clouds, water vapor, aerosols). These two numbers cannot be compared at their face value.”
Why can’t you compare the two numbers? Radiative forcing refers to a disturbance in the planet’s energy balance. Forcings change the balance between incoming sunlight and outgoing radiation at the top of the atmosphere, causing the planet to lose or gain energy. Global temperatures will only respond to surface brightening if the total amount of solar energy absorbed by our climate system changes. To determine this, we need to understand what’s causing global brightening.
There are three major contributors: a reduction in cloud cover, a reduction in scattering aerosols such as sulfates and a reduction in absorbing aerosols like soot (Wild 2009). Scattering aerosols reflect incoming sunlight, preventing it from reaching the Earth’s surface. As the amount of sulfate pollution in the atmosphere lessens, more sunlight reaches the surface. If this was the sole cause of global brightening, then the increase in surface solar radiation would equal the extra energy absorbed by our climate (eg – the radiative forcing).
However, changes in cloud cover and absorbing aerosols also contribute to global brightening. As well as reflect sunlight, clouds trap infrared radiation coming up from the surface. So while less clouds allow more sunlight to reach the surface which has a warming effect, they also let more infrared radiation escape to space which has a cooling effect.
Similarly, a decline in absorbing aerosols like soot means more sunlight reaches the Earth which has a warming effect. But they also absorb sunlight which warms the atmosphere so a decline in absorbing aerosols also has a cooling effect. Absorbing aerosols like black carbon have shown a large decreasing trend since the 1980s (Wild 2009).
To focus solely on the amount of sunlight hitting the Earth doesn’t give you the full picture of global brightening. As absorbing aerosols and clouds are contributing factors, the change in surface solar radiation is expected to be much more than the net radiative forcing from global brightening. To gain a fuller understanding of climate, we need to consider all the various forcings together rather than take one small piece in isolation. These include the direct effect from reflective aerosols, the indirect effect of aerosols on cloud cover and the effect of absorbing aerosols (black carbon) to name just a few.
Figure 2: Separate global climate forcings relative to their 1880 values (GISS).
Figure 2 demonstrates that CO2 is not the only driver of climate. Nevertheless, it’s clear that man-made greenhouse gases (of which CO2 is the greatest contributor) is currently the most dominant forcing and increasing faster than any other forcing.