Many of the observed changes are unprecedented, particularly since 1950s. Changes in the large-scale atmospheric circulations, e.g., Walker Circulation (WC) and tropical belt width, can be attributed to natural variability and anthropogenic forcing. This raises the question about the contribution of natural variability and anthropogenic forcing to observed changes. I used climate modelings to investigate the dominant driver of recent observed changes, focusing on the WC and tropical belt expansion.
The tropical belt has observed to widen over the past few decades. Natural variability, greenhouse gas, aerosol and stratospheric ozone depletion have been suggested to be important contributor to the tropical expansion. The net influence of natural variability and anthropogenic forcers on tropical belt width variations are complicated and contains large uncertainty, closely associated with aerosols. I used idealized PDRMIP simulations to investigate the response of tropical belt width to different forcers. Results show that absorbing black carbon (BC) aerosol drives tropical expansion and scattering sulfate aerosol drives contraction. Tropical belt expansion (contraction) is associated with an increase (decrease) in extratropical static stability induced by absorbing (scattering) aerosol. Via linear attribution, result shows that BC might the largest driver of historical Northern Hemisphere tropical belt widening.
Climate models simulate a weakening of WC under the warming, however, I found WC intensified based on observations and multiple reanalyses since 1979. Atmosphere-only simulations driven by the real-world evolution of sea surface temperatures (SSTs) simulate the observed intensification of WC, whereas coupled ocean atmosphere simulations do not. Idealized simulations driven by the unforced component of SSTs yield significant WC strengthening, whereas negligible WC changes are simulated driven by the forced SST component. These findings suggest that the recent strengthening of the WC since 1979 is attributed to natural SST variability, particularly associated with a La Niña-like SST pattern.
Anthropogenic forcers-aerosols, are also generally considered to be the major cause of air pollution. Current air quality guidelines target man-made air pollutants, while the contributions of natural aerosols are of less importance. However, natural aerosols have strong geographic gradients and this suggests that spatially invariant air quality guidelines may handicap regions close to natural sources. Climate models are used to to construct a view of pre-industrial ``pristine" air quality. Compared to the World Health Organization's globally uniform air quality thresholds, pristine sources of aerosols lead to poor air quality induced by natural (i.e., pristine) pollution alone, particularly near dusty regions, estimated nearly 1 billion people impacted. World Health Organization lowers fine particular annual mean threshold in 2021. Based on the latest air quality guideline, estimated impacted population rises to ~3.6 billion. These findings imply that air quality guidelines established based on anthropogenic metrics unfairly bias countries close to natural sources.
