Saturday, July 18, 2015

The Future of Earth - By Fire

Will it be by fire or by ice that Earth will meet its end? Well it all depends on how you define "end". It has been known for quite some time that eventually the Earth will be swallowed by an expanding sun when the sun reaches its final stages of life as a red giant. This stage will begin sometime between 5 and 7 billion years from now - here is a nice video giving some great visualizations of what we think will happen. But a recent article in the Journal of Geophysical Research - Atmosphere tells us that we'll have our last best day far sooner. 

The sun is getting brighter (see footnote 1). One of the early challenges for paleoclimatologists (people who study the climates of yesteryear) was understanding the evolution of sun like stars and its impact on climate early in life's history on Earth. In a recent article in the journal JGR-Atmospheres authors Wolf and Toon found that increasing solar luminosity will make the Earth uninhabitable in about 2 billion years (time to start planning folks!). The good news is that though life won't be able to survive, the planet is unlikely to experience a runaway greenhouse effect as may have been the case of Venus.

Somewhat more seriously it is not that clear in my skimming the article (apologies if it is covered - time constraints) if surface albedo might influence the evolution of Earth's climate under a brightening sun. Here is where we find the theories of James Lovelock and the Gaia Hypothesis

1 From Paleoclimate Implications for Human-Made Climate Change (pdf)
James E. Hansen and Makiko Sato

Solar luminosity is increasing on long time scales, as our sun is at an early stage of solar evolution, "burning" hydrogen, forming helium by nuclear fusion, slowly getting brighter. The sun's brightness increased steadily through the Cenozoic, by about 0.4 percent according to solar
physics models (Sackmann et al., 1993). 

Urban Climate

A recent journal article by MZ Jacobson et al points out some of the climatological impacts of urbanization on local climate. The land use change that urbanization leads to is known to impact climatological conditions through changes in soil moisture evaporation, transpiration (evaporation from plants), heat absorption and advection (wind).

In this article the authors used models to study the impact of the change in urban extent in Beijing. Beijing provides a good case due to the extreme nature of changes there - the urban extent of the city quadrupled between 2000 and 2009.

Key Points from the article:

  • Beijing's expansion created a ring of impact in the new portion of the city
  • Without considering the impact of more vehicles and other sources of human caused polution - urbanization's impact on climatological conditions alone slowed winds and increased pollution vertical dilution and increased ground level temperature, and ozone

Interestingly the authors used crowdsourced data on road surface area in their study.

Take homes:
This study found that urbanization changes local climate in these ways - it:

  • increases ground level temperature
  • decreases ground level humidity
  • decreases horizontal movement of air (wind, advection)
  • increases vertical movement of air (convection)
  • reduces reflectivity of the surface (albedo) causing the retention of more solar energy
These impacts:

  • decreased surface pollution by promoting vertical mixing
  • but increased surface ozone (O3) due to other changes in the chemical profile of the air
None of these impacts takes into account vehicle use or other pollution sources.

Mark Z. Jacobson, Son V. Nghiem, Alessandro Sorichetta and Natasha Whitney
Article first published online: 19 JUN 2015 | DOI: 10.1002/2014JD023008