Is Amazon Rainfall Changing?

This is a look at ERA rainfall data for the Amazon region (as I define on the photo below). The area is a proxy for the Amazon Basin, meaning that it doesn’t precisely conform to the basin but it does encompass most of the Basin and not a lot of non-Basin area.

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Ecologists and others are concerned about the effects of deforestation in the Amazon region. So, just for exploration, I’ll plot both rainfall and deforestation.

Let’s start with basics. Here are plots of monthly rainfall since 1979 and of rainfall by month of the year:

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Rainfall in the Amazon region has trended upward over the last 35 years. The trend has been irregular but substantial – average rainfall today is about 20-25% higher than 35 years ago, based on this ERA data. Wow.

Using the region’s average rainfall and size, and assuming that 78 W/m3 of energy leaves Earth via precipitation processes (per IPCC), about 1.4 W/m3 of global heat removal is associated with the Amazon region rainfall. A 25% growth in rainfall means that 0.35W/m3 has been added to global heat removal (everything else being equal, which might or might not be true).

The next plot shows rainfall by month. It indicates substantial intra-annual variation (wet season and dry season), possibly due to the annual travels of the ITCZ (intertropical convergence zone) across the region.

0131153Let’s see if the 35-year trend varies by wet season and dry season. Below is a plot of the trend for the average of the wettest three months and of the driest three months. It shows that both seasons have gotten wetter , with the wet season’s growth happening a little faster than the dry season’s.

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That variation over the year deserves a little closer look – is the rainfall increase only a wet-season phenomenon? Below is a plot of the slope of the linear trendlines for each month of the year. There is a clear seasonality to the rates of change, with the wet-season showing the greatest rates of change. That makes me wonder if the ITCZ (to which I attribute the wet season, to be explored) is more active nowadays than in the past. If so, why?

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Now, a look at deforestation.  Below, in red, are estimates of the decline in Brasilian Amazon forestation as a percent of 1960s forest cover. The plot also shows rainfall trends in the Amazon region. All that can be said from this is that forest coverage declined while rainfall trended upwards: correlation of trends is not necessarily causation.

There’s a considerable amount of peer-reviewed literature on the possible impacts of deforestation and climate change, much of it based on modeling. One paper speculates that reduced forest cover means reduced transpiration and thus less water vapor. Others consider the possibility that there might be an impact on ground-level wind or albedo. I believe that we’re far from knowing the answer.

If there actually is a relationship (less forest cover leads, say, to a wetter wet season) then the latent heat released by that greater rain might be large enough to alter regional weather patterns elsewhere on the globe.

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