Changes in external forcing, which can push Earth toward warming or cooling, could also be affecting recent GMST trends. It is well established that the long-term warming trend in the climate is caused mainly by increasing atmospheric concentration of human-caused greenhouse gases (GHGs). In addition, the rate of warming is affected by stratospheric aerosols produced from volcanic eruptions and manmade pollution, stratospheric ozone, tropospheric aerosols, solar irradiance, and water vapor content in the atmosphere (Schmidt et al., 2014). In considering the potential contribution of changes in external forcing to the slowdown, some participants made the point that global ocean heat content does not reveal an obvious slowdown in the warming trend in the past decade. If one considers ocean warming as a proxy for radiation imbalance, then the continued warming suggests that the amount of radiative imbalance at the top of the atmosphere may have remained unchanged between the 1990s and the 2000s, arguing against a significant effect of changes in external forcings on GMST.
Susan Solomon from the Massachusetts Institute of Technology provided a brief survey of radiative forcing changes during 2000-2014 that might have contributed to the decadal rates of global surface warming. Solomon compared Earth’s radiative budget over the long term (1750-2009) to the short term (1999-2009) to show the differences in contributions from various sources (see Figure 15). She said that there is much more confidence now in measures of volcanic forcing. In contrast, there is high uncertainty in the data on tropospheric aerosols, and stratospheric H2O may be a feedback that is not accurately represented in models.
Volcanic eruptions may have significantly contributed to recent variability. Satellite data that were not available prior to the early 2000s show the influence of a series of eruptions after 2005 (Solomon et al., 2011; Ridley et al., 2014). A significant cooling (a few tenths of a degree) of tropical SSTs after the Nabro volcanic eruption in 2011 suggests a forced component to observed tropical sea surface temperature (SST) cooling (Santer et al., 2015).
Solomon highlighted several reasons why the forcing from volcanic eruptions might be underestimated in models. Her own study of the optical depth of aerosols (a measure of the amount of light lost due to the presence of aerosols) concluded that the optical depth value post-2000 never reached zero, indicating that some aerosols were always present (Solomon et al., 2011). However, volcanic aerosols were set too low in Coupled Model Intercomparison Project Phase 5 (CMIP5) models, according to Solomon, which amounted to a net positive forcing in the models. Inclusion of more realistic forcing from background aerosols in the models has been shown to account for up to one-third of the recent GMST slowdown trend, or 0.05 C (Solomon et al., 2011).
In addition, recent work by Ridley et al. (2014) showed that satellite data do not measure the area between 15 km and the tropopause over the extratropics where a significant amount of volcanic material resides—as much as three times as what has been observed above that region by satellites. Thus, if LIDAR1 data and weather balloons were used to fill in the data in that region, aerosol values would increase significantly (Ridley et al., 2014). Promising new methods have been developed to study the tropopause, which has also been difficult to observe. However a systematic plan to improve observations there is needed, said Solomon.
Stratospheric water vapor also could be playing a large role in GMST variability, according to Solomon. Limited data before the mid-1990s suggest that stratospheric water vapor increased up to 2000, which could be an important factor in the accelerated warming from 1980 to 2000 (Forster and Shine, 1999). Stratospheric water vapor significantly dropped after 2000 and stayed low through much of the 2000s (Figure 16). Average forcing for 2005-2014 from observed stratospheric water vapor changes has been estimated at -0.04 W/m2 (Gilford et al., 2015), which might represent as much as 25 percent of the total forcing from all factors during that time, according to Solomon.
Regarding solar forcing, Solomon said that a symmetric cyclic forcing has little or no net effect on Earth’s energy budget. However, from 2000 to 2012, solar forcing was not a symmetric cycle. Hansen et al. (2011) estimated that there was about -0.1 W/m2 from solar forcing over that period. The solar drivers in the CMIP5 simulations were overestimated because the last solar-cycle minimum was lower and the present weak solar cycle started later than assumed at the time, which may have contributed to the models’ projection of warming (Schmidt et al., 2014).
1 Light Detection and Ranging (LIDAR) is a surveying technology that measures distance using laser light.
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