# Contents

## Idea

From Wikipedia:

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth’s surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.It is thought to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action. The switch from a “global dimming” trend to a “brightening” trend in 1990 happened just as global aerosol levels started to decline. Global dimming has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming. Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of global warming.

Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise. According to Beate Liepert, “We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter.” The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.

## References

Creative Commons or free access

Abstract: Understanding the sensitivity of Earth’s climate to an imposed external forcing is one of the great challenges in science and a critical component of eﬀorts to avoid dangerous anthropogenic interference with the climate system. Climate sensitivity (or equilibrium global surface warming) to a doubling of atmospheric CO2 has long been estimated to be about 3◦C, considering only fast climate feedbacks associated with increases in water vapor, decreases in sea ice, and changes in clouds. However, evidence from Earth’s history suggests that slower surface albedo feedbacks due to vegetation change and melting of Greenland and Antarctica can come into play on the timescales of interest to humans, which could increase the sensitivity to signiﬁcantly higher values, as much as 6 ◦C.

Even higher sensitivity may result as present-day land and ocean carbon sinks begin to lose their ability to sequester anthropogenic CO2 in the coming decades. The evolving view of climate sensitivity in the Anthropocene is therefore one in which a wider array of Earth system feedbacks are recognized as important. Since these feedbacks are overwhelmingly positive, the sensitivity is likely to be higher than has traditionally been assumed.

Abstract:In this study the direct and diffuse solar radiation changes are estimated, and they contribute to the understanding of the observed global dimming and the more recent global brightening during the industrial era. Using a multistream radiative transfer model, the authors calculate the impact of changes in ozone, $NO_2$, water vapor, $CH_4$, $CO_2$, direct and indirect aerosol effects, contrails, and aviation-induced cirrus on solar irradiances at the surface. The results show that dimming is most pronounced in central Africa, Southeast Asia, Europe, and northeast America. Human activity during the industrial era is calculated and accounts for a decrease in direct solar radiation at the surface of up to 30 W m−2 (30%–40%) and an increase in diffuse solar radiation of up to 20 W m−2. The physical processes that lead to the changes in direct and diffuse solar radiation are found to be remarkably different and the authors explain which mechanisms are responsible for the observed changes.

Not free

Global dimming and brightening is a popular expression which refers to the recent observational evidence for substantial decadal variations in solar radiation reaching the Earth’s surface. For the first time, a special section is dedicated to this rapidly growing field of research. A comprehensive collection of more than 20 papers sheds new light on the phenomenon of global dimming and brightening. Here I provide a brief introduction and guideline to the special section and place the individual papers into context. To facilitate orientation in this special section, the papers are grouped according to the specific aspects of global dimming and brightening that they address.

Abstract: Observational evidence indicates significant regional trends in solar radiation at the surface in both all‐sky and cloud‐free conditions. Negative trends in the downwelling solar surface irradiance (SSI) have become known as ‘dimming’ while positive trends have become known as ‘brightening’. We use the Met Office Hadley Centre HadGEM2 climate model to model trends in cloud‐free and total SSI from the pre‐industrial to the present‐day and compare these against observations. Simulations driven by CMIP5 emissions are used to model the future trends in dimming/brightening up to the year 2100.

The modeled trends are reasonably consistent with observed regional trends in dimming and brightening which are due to changes in concentrations in anthropogenic aerosols and, potentially, changes in cloud cover owing to the aerosol indirect effects and/or cloud feedback mechanisms. The future dimming/brightening in cloud‐free SSI is not only caused by changes in anthropogenic aerosols: aerosol impacts are overwhelmed by a large dimming caused by increases in water vapor. There is little trend in the total SSI as cloud cover decreases in the climate model used here, and compensates the effect of the change in water vapor.

In terms of the surface energy balance, these trends in SSI are obviously more than compensated by the increase in the downwelling terrestrial irradiance from increased water vapor concentrations. However, the study shows that while water vapor is widely appreciated as a greenhouse gas, water vapor impacts on the atmospheric transmission of solar radiation and the future of global dimming/brightening should not be overlooked.

category: climate