Isolating the roles of different forcing agents in global stratospheric temperature changes using model integrations with incrementally added single forcings

Abstract

Satellite instruments show a cooling of global stratospheric temperatures over the whole data record (1979-2014). This cooling is not linear, and includes two descending steps in the early 1980s and mid-1990s. The 1979-1995 period is characterized by increasing concentrations of ozone depleting substances (ODS) and by the two major volcanic eruptions of El Chichón (1982) and Mount Pinatubo (1991). The 1995-present period is characterized by decreasing ODS concentrations and by the absence of major volcanic eruptions. Greenhouse gas (GHG) concentrations increase over the whole time period. In order to isolate the roles of different forcing agents in the global stratospheric temperature changes, we performed a set of AMIP-style simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). We find that in our model simulations the cooling of the stratosphere from 1979 to present is mostly driven by changes in GHG concentrations in the middle and upper stratosphere and by GHG and ODS changes in the lower stratosphere. While the cooling trend caused by increasing GHGs is roughly constant over the satellite era, changing ODS concentrations cause a significant stratospheric cooling only up to the mid-1990s, when they start to decrease because of the implementation of the Montreal Protocol. Sporadic volcanic events and the solar cycle have a distinct signature in the time series of stratospheric temperature anomalies but do not play a statistically significant role in the long-term trends from 1979 to 2014. Several factors combine to produce the step-like behavior in the stratospheric temperatures: in the lower stratosphere, the flattening starting in the mid 1990's is due to the decrease in ozone depleting substances; Mount Pinatubo and the solar cycle cause the abrupt steps through the aerosol-associated warming and the volcanically induced ozone depletion. In the middle and upper stratosphere, changes in solar irradiance are largely responsible for the step-like behavior of global temperatures anomalies, together with volcanically induced ozone depletion and water vapor increases in the post-Pinatubo years.

Observation and Attribution of Temperature Trends Near the Stratopause From HALOE

“…HALOE T(p) shows a minimal cooling of − 0.2 K/decade at 1 hPa, as opposed to the maximum radiative response of Figure for GHG and for ozone when ODS were increasing in 1980–2000. These differences agree qualitatively with the conclusion of Aquila et al () that changing from a negative to a slightly positive ozone trend is the primary radiative forcing agent for the corresponding changes in global temperature trends from 1979–1997 to 2000–2011.…”

State of the Climate in 2016

“…Pinatubo (1991) but shows little trend since 1994. Offsetting impacts of increasing ozone (positive temperature anomalies) and increasing CO2 (negative temperature anomalies) in the lower stratosphere may have led to this flat trend (Ferraro et al 2015;Aquila et al 2016). The various observation systems differ slightly from each other during this period since 1994.…”

Radiosondes Show That After Decades of Cooling, the Lower Stratosphere Is Now Warming

“…In the lower stratosphere, changes in ODSs have a clearly detectable influence on ozone. Other influences (such as volcanic effects) are also detectable (Aquila et al, ; Bandoro et al, ; Gillett et al, ; Polvani et al, ).…”