The Globe’s Total Greenhouse Effect Forcing Has Been On A Declining Trend For Decades

CO2’s impact on the total net greenhouse effect (GHE) or longwave (LW) forcing trend has been offset by the dominant impact of cloud cover since the 1980s. 

It is now widely accepted that changes in clouds, naturally driven by changes in atmospheric circulation, “may be the most important parameter controlling the radiation budget, and, hence, the Earth climate” (Sfîcă et al., 2021). Indeed, in recent decades, the post-1980 +3 W/m² increase in absorbed solar radiation due to the reduction in cloud albedo (Delgado-Bonal et al, 2020) can explain the warming in recent decades.

Image Source: Sfîcă et al., 2021

An overlooked 2020 observational study (Su et al., 2020) suggested that while top of the atmosphere (TOA) shortwave (SW) forcing has risen at a rate of +0.61 W/m² per decade since the early 2000s (because “decreasing cloud optical depth…causes an overall increase in TOA SW flux”), the total longwave (LW) or greenhouse effect (GHE) forcing – which includes the total net forcing impact of CO2 – has declined at a rate of -0.17 W/m² per decade.

Image Source: Su et al., 2020

These trends are consistent with Kato et al., 2018. They found the total downwelling SW forcing (“downward shortwave irradiance”) rose +1.3 W/m² from 2005-2014 (and +2.2 W/m² per decade from 1986-2000). In contrast, the total LW (greenhouse effect) forcing from clouds, water vapor, and CO2 combined to elicit a trend of -0.2 W/m²  per decade from 2005-2014.

Image Source: Kato et al., 2018

Cess et al. (2003) also explained total greenhouse effect forcing declines as the absorbed solar radiation increases due to the “major impact” of clouds. Decadal-scale cloud cover changes are “the result of natural variability on decadal time scales,” or “internal unforced variability.”

Not only is the negative trend in cloud fraction consistent with the positive trend in ASW [absorbed shortwave], it is also consistent with the negative trend in G [greenhouse effect anomaly]” that occurred from 1985-1999.

Image Source: Cess et al., 2003

If the net impact of greenhouse effect forcing has been negative since the 1980s while the trend in absorbed SW forcing has been positive, this explains the warming trends in the last decades and precludes an attribution for anthropogenic greenhouse gas emissions.

4 responses to “The Globe’s Total Greenhouse Effect Forcing Has Been On A Declining Trend For Decades”

  1. S. K. Dodsland

    Beside the variability of the radiation data, the credibility of the temperature data is a huge question mark.

    NASA/NOAA are altering the North American temperature data making it unusable.

  2. Ulric Lyons

    Weaker solar wind states since 1995 have driven warmer ocean phases, which have driven a decline in low cloud cover. The 1970’s had the strongest solar wind conditions of the space age, driving colder ocean phases.

  3. Anders Valland

    I live in the middle of Norway, some 700 km south of the arctic circle. Around Christmas we do get some 6 hours of daylight, right now we have 8 hours between sunrise and sunset.

    We are in the middle of a cold spell, with light to no winds. We have had this situation since early December, and that gives some interesting observations.

    In December we had no snow on the ground, and no precipitation. Air temperature was consistently high for the season, but whenever we had no wind and clear skies temperature would go to -1 to -2 degrees Celsisus. With clouds it would rise to +3 to +5 degrees.

    During Christmas we had light snow, and for the past 4 weeks we have had snow cover on the ground. We have for the past 2 weeks had clear skies and little to no wind, and temperatures have been steady at -10 to -14 degrees. The past 3 days have been overcast, and temperatures rose to and stabilized at -5 to -7 degrees.

    That is the effect of clouds. We get very little heat from the sun in the middle of winter, so air temperature close to the ground is governed by radiative loss.

    I live very close to a fjord, less than 1 kilometre line-of-sight, and that modifies temperatures quite a bit. Move inland, and the difference in air temperature between clear and cloudy is larger.

  4. Weekly Climate and Energy News Roundup #442 – Watts Up With That?

    […] The Globe’s Total Greenhouse Effect Forcing Has Been On A Declining Trend For Decades […]

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