Climate Change and its Impact on the World’s Cities

While I realize that there are many sceptics out there when it comes to the anthropogenic causes of global climate change, I have one question:

"What if climate change is real and is due to the activities of man since the beginning of the Industrial Age?"

A recent study by Jean-Francois Bastin et al entitled "Understanding Climate Change from a Global Analysis of City Analogues" takes a very interesting view of how climate change (if it is real and will proceed as projected) will impact iconic cities around the world.  The study looks at whether 520 cities around the world will have a climate that is similar to their current climate or whether it will more closely resemble the climate of other major cities in 2050, three short decades from now.  

One of the biggest problems facing climate scientists is the fact that potential future climate data does not have any impact on human beliefs, particularly when those scientific projections are measured in terms of very lengthy periods of time (i.e. the year 2100) or relatively abstract concepts like sea level increases measured in centimetres.  The authors of the study note that "geographic shifts" are the most effective way of communicating the impact of climate change, that is, comparing the climate of cities today to what the climate will be like in the future by using current climatic conditions in other cities.  This type of climate change measurement is referred to as "city analogues". 

Here is a quote from the paper explaining the concept of city analogues and what the authors hope to achieve with this study:

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"In this study, we evaluate the global shifts in the climate conditions of cities by taking current climate data for the world’s 520 major cities (Current Cities with populations in excess of one million people), and project what they will most closely resemble in 2050 (Future Cities). Rather than describing the quantitative changes in climate variables, we propose to quantify city climate analogs at a global scale, i.e. assessing which Current Cities will most closely resemble the climate conditions of Future Cities. To tackle previous limitations, we explore these patterns at a global scale using 19 bioclimatic variables, to include climate variability and seasonality in addition to climate averages.

Specifically, we aim to test three questions:

(i) What proportion of the world’s major cities of the future most closely resemble their own current climate conditions vs. the climate conditions of other cities in different geographic regions? 

(ii) What proportion of cities will experience novel climate conditions that are outside the range experiences by cities today? 

(iii) If cities do shift their climate conditions, is this spatial shift uniform in direction across the planet?

The following principal components were examined:

1.) minimum temperature of the coldest month.

2.) maximum temperature of the warmest month.

3.) precipitation seasonality.

4.) precipitation of the driest and wettest months.

5.) temperature diurnal range.

Here is a plot showing the distribution of current and future cities along the four principal component axes:

The top figure shows the distribution of current and future cities with the first axis being driven mainly by the differences in temperature seasonality and minimum temperature of the coldest month which is responsible for 40.2 percent of climate variation.  The second axis shows precipitation seasonality which is responsible for 26.9 percent of climate variations.  The bottom figure shows the distribution of current and future cities with the first axis showing changes in precipitation of the wet season which is responsible for 10.5 percent of climate variation.  The second axis is driven mainly by changes in the mean diurnal temperature range which is responsible for 7.6 percent of climate variation.

The authors then calculated the latitudinal (i.e. north/south) shift between the current and future cities.  They noted that there is less temperature seasonality with higher maximum and minimum temperatures and well as higher precipitation seasonality with higher amounts of precipitation in the wettest months and lower precipitation in the driest months.  To summarize:

"...cities of the world will become hotter, particularly during the winter and summer.  Wet seasons become wetter and dry seasons drier."

Let's look at additional details on what the authors of the study found.  Their analysis shows that, in 2050, 23 percent of the world's major cities will have a climate that is similar to their current climate and 77 percent of cities will experience a significant change in climate, resulting in climate conditions that are similar to today's climate in other cities.  Additionally, 22 percent of the world's cities are likely to exist in a climate domain that does not exist today.  Tropical regions will see the greatest change with 30 percent of cities experiencing a drier climate, a climate condition that does not exist today.  Here is a graphic showing the extent of climate change with cities that will experience climate conditions that no major city has experienced before coloured in red and those that will experience climate conditions that are reflected in current major cities today ( noting that the larger the dot, the greater the magnitude of change): 

Here is a graphic showing the latitudinal shift in climate in 2050 versus the equator for the world's major cities noting that the climate of northern hemisphere cities shift to climates that resemble the climate closer to the equator:

Let's look more closely at the shift in climate by 2050.  Cities in northern latitudes will experience the  most dramatic shifts in extreme temperature with cities in Europe seeing warmer summers with temperatures increasing 3.5 degrees Celsius and warmer winters with temperatures increasing 4.7 degrees Celsius.  This is equivalent to moving a European city 1000 kilometres closer to the equator.  Madrid's climate will be more like that of Marrakech, London's climate will be more like that of Barcelona, Stockholm's climate will be more like that of Budapest, Portland (Oregon) will have a climate that is more like San Antonio and San Francisco will have a climate that is similar to Lisbon.

Let's close with this graphic which shows the geographical shift of the climate (in degrees) toward the subtropics (i.e. 20 degrees latitude):

Cities located below 20 degrees north and south tend to move away from the equator while cities that are beyond the 20 degree latitude line tend to move toward the equator (i.e. the climate becomes more subtropical).

Here is a summary of the findings as quoted in the paper:

"At the global scale, our study reveals that geographical shift tend to converge towards the subtropics, going to warmer climate conditions from boreal and temperate regions and to drier conditions from tropical regions. While this lends support to previous observations of a “tropical belt widening” due to the expected warmer conditions, it also shows that tropical biomes tend to shrink in many areas due to drier conditions. We therefore suggest here to refer to a “sub-tropical widening” compared to the previous “tropical widening” due to climate change….

To our knowledge, our study represents the first global analysis of the shifts in climate conditions of the world’s major cities under climate change. Our analysis revealed that over 77% of the world’s cities are likely to experience a shift towards the climate conditions of another major city by 2050, while 22% will shift to climate conditions that are not currently present for any major cities on the planet. Across the globe, the direction of movement is generally trending towards the subtropics, providing unifying patterns that support trends observed in Europe and North America. In addition, this analysis revealed new insights for cities in equatorial regions, many of which are likely to move to entirely new climate conditions that are not currently experienced by any of the other global cities today. These city analogues, and the data we openly share, can help land managers and city planners to visualize the climate futures of their respective cities, facilitating efforts to establish targeted climate response strategies." (my bolds)

As I stated in my opening; what if?

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