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In the past, the effects of climate-change have largely been climatological. There are many types of records such as global temperature rise, increase in sea level, warming oceans, shrinking ice sheets, declining arctic sea ice, retreating glaciers, and ocean acidification that prove it is happening [1]. We also know that there has been an increase in extreme weather events. However, attributing these events to climate change has been elusive. It has largely been limited to attributing the effects climate-change has on the thermodynamics of the atmosphere. The most attributable events are more frequent heat waves, and fewer cold snaps [2]. Higher temperatures can also lead to more extreme precipitation events as a result of the water-holding capacity of the atmosphere going up exponentially at a rate of about 7% per degree Celsius [3]. The increase heating can also lead to higher levels of evaporation and evapotranspiration intensifying droughts.

Many regions in the United States have experienced less than 1 degree Fahrenheit (0.55 degrees Celsius) warming over the last century, with some of them experiencing a decrees in temperature [Fig. 1]. This makes it difficult to attribute increased temperatures to many of the extreme weather events that have occurred in these regions. It may also explain some of the difficulty in convincing people, in these regions, of the existence of global warming, since they are not personally experiencing the warming. However, the disbelief in the existence of global warming probably has more to do with their political view [4], than any direct experiences. Some of these regions have experienced hurricanes which were intensified by higher sea surface temperatures (SST), and more intense rainfall due to more moisture in the atmosphere, but thermodynamics only tells party of the story of what is happening with extreme weather in these regions.

Figure 1

Atmospheric circulation is extremely important when attributing extreme weather events, but connecting changes in atmospheric circulation to climate-change is much more difficult [5]. One of the difficulties is our ability to accurately model atmospheric dynamics over long periods of time. In is seminal 1963 paper "Deterministic Nonperiodic Flow" [6], Lorenz laid the foundations for chaos theory [7] showing that small differences in initial state can evolve into considerable different states. Atmospheric circulation has an inherent chaotic quality, but overtime patterns emerge.

The most obvious and best understood patterns are seasonal, which are driven by changes in the length of day and position of the Sun in the sky, due to Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. There are also inter-annual patterns such as El Nino southern oscillation (ENSO) [8] which occur every few years, and patterns that occur over decades such as the Pacific decadal oscillation [9]. These patterns are linked to shifts in predominant air pressure associated with changes in sea surface temperature SST. In the case of ENSO increased SST in the eastern tropical Pacific are associated with shifts in air pressure between Darwin Australia and Tahiti. Due to the scale of the Pacific ENSO’s impact is dramatic, shifting weather patterns far outside of the tropics.

An “Arctic amplification” (AA) is occurring as a result of the Arctic warming twice a fast as lower latitudes [10]. This is causing a decrease in temperature difference between the Arctic and mid-latitudes that subsequently causes the jet stream to slow, and to become wavier [Fig. 2] due to its decreasing speed [11]. The wavier jet stream has been linked to severe weather events in the mid-latitudes, including heat waves, droughts, extreme precipitation and heavy snowfalls. When the waives get large they tend to slow creating more persistent weather patterns in lower latitudes. The wavier jet stream is also contributing to AA by transporting more water vapor, a greenhouse gas (GHG), from lower latitudes into the Arctic. AA is also a result of less sea ice cover and thinner sea ice, resulting in more evaporation from the ocean surface, increasing the water vapor in the atmosphere. AA has similarities to the Arctic oscillation (AO) [12] but is more complex and persistent. Its influence on weather at lower latitudes is an example of how climate change can impact atmospheric circulation, and in turn enable Scientist to potentially attribute specific weather events to climate change.

Figure 2

References



  1. “Climate change, How do we know?”, NASA

  2. “Attribution of Extreme Weather Events in the Context of Climate Change”, Committee on Extreme Weather Events and Climate Change Attribution; Board on Atmospheric Sciences and Climate; Division on Earth and Life Studies; National Academies of Sciences, Engineering, and Medicine, 2016

  3. Kevin E. Trenberth, John T. Fasullo, Theodore G. Shepherd. “Attribution of climate extreme events”, Nature Climate Change, 2015

  4. Anthony Leiserowitz, Edward Maibach, Connie Roser-Renouf, Matthew Cutler and Seth Rosenthal. “Trump Voters & Global Warming”, Yale Program on Climate Change Communication, 2017

  5. Theodore G. Shepherd. “Atmospheric circulation as a source of uncertainty in climate change projections”, Nature Geoscience, 2014

  6. Edward N. Lorenz. “Deterministic Nonperiodic Flow”, Journal of Atmospheric Science, 1963

  7. Chaos Theory, Wikipedia

  8. El Nino, Wikipedia

  9. Pacific decadal oscillation, Wikipedia

  10. Major Report Prompts Warnings That the Arctic Is Unraveling, Scientific America

  11. The Arctic Is Getting Crazy, Scientific America

  12. Arctic Oscillation, Wikipedia


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