To address this, researchers from the University of Reading, UK, applied a simple model to examine the role of individual greenhouse gases from 1850–2014. Before assessing the different greenhouse gas contributions to the recent slowdown, the scientists first compared results from their energy-balance model with outputs from more sophisticated global climate models. Once satisfied that their tool reproduced major global-mean surface temperature characteristics such as the hiatus between 1945 and 1960 and the slowdown around the year 2000, the researchers zoomed in on the contribution of individual greenhouse gases.
The team focused in detail on the period from 1985 to 2003. "These data highlight that slowing trends in ozone-depleting substances and methane have made a substantial contribution to the slowdown in global-mean surface temperature around 2000," Ramiro Checa-Garcia told environmentalresearchweb.
The work provides evidence of the success of the Montreal Protocol in curbing the emission of ozone-depleting substances, which are also strong greenhouse gases, into the atmosphere. "The protocol not only protected the ozone layer, but also led to a discernible contribution to the recent slowdown in global warming," said Michaela Hegglin, who also contributed to the analysis.
The team’s results suggest that the effect of ozone-depleting substances on the slowdown in the rate of increase of global-mean surface temperature has essentially finished – following the transition from a rapid increase to a slow decline, which is due to the long lifetime of these substances in the atmosphere.
In addition, the work draws attention to the weakening in growth rates of methane and tropospheric ozone radiative forcing over time, which has helped to mask the full effect of carbon dioxide. However, it is interesting to note that after 2000 methane no longer contributes to the reduction in the global-mean surface temperature trend, according to supplementary results supplied by the group.
"The poorly understood factors that led to a slowdown in the growth of methane emissions appear to be no longer acting, as methane has now resumed its upward trend," added Reading researcher Keith Shine.
The group concludes that unless there is an unexpected rapid decline in tropospheric ozone radiative forcing, or a reversal of recent upward methane trends, it can be expected that, as a whole, greenhouse gases will cause an acceleration in global-mean surface temperature trends in the near future.
Full details on the work can be found in Environmental Research Letters (ERL).
Related links
- The contribution of greenhouse gases to the recent slowdown in global-mean temperature trends R Checa-Garcia et al 2016 Environ. Res. Lett. 11 094018
- ERL
- Ramiro Checa-Garcia
- Michaela Hegglin
- Keith Shine