In our previous article we saw how climate change dries out the Amazon rainforest from the South – killing all remaining rainforest in Bolivia and Paraguay, and most in Peru and Brazil.

So, we wonder, what’s going on with the rainforests further to the North? Are these more resilient? Well, the northern margin of the Amazon basin: perhaps – but Central America: probably not – a recent study says.

Clear decrease in average rainfall over Central America as a consequence of 21st century climate change, depending on the height of cumulative greenhouse gas emissions.

Central America deserves special attention in our series. The isthmus (‘land bridge’) forms an ecological corridor between North and South America – that is geologically speaking not that old (it formed in the Pliocene, some 3 million years ago). Therefore the region first connected and then separated Atlantic and Pacific marine biodiversity, and first separated then connected American terrestrial biodiversity. Resulting from these special dynamics the region is recognised as an official biodiversity hotspot, called the Mesoamerican biodiversity hotspot.

Effects of warming on Mesoamerican rainforests

The new study, titled ‘Projections of climate change impacts on central America tropical rainforest’ was published in March 2017 in the journal Climatic Change by a team of researchers of the Brazilian National Institute for Space Research (INPE) and the Tropical Agricultural Research and Higher Education Center of Costa Rica (CATIE).

The South Americans used a number of climate and vegetation models to examine the effects of anthropogenic climate change under two established global greenhouse gas emission scenarios, RCP4.5 and RCP8.5 – the first being “worse than the 1.5 pledge of Paris but better than the actual INDC commitments of countries”, the second representing a no-policy ‘business as usual’ increasing emissions world.

Both scenarios show significant (above global average) warming in the region. The used climate model (Eta-HadGEM2-ES) projects a warming between 3 to 3.5 degrees Celsius under RCP4.5 emissions and 6 to 7 degrees under RCP8.5*.

[*) That’s a really rapid warming under both scenarios, considering the baseline (1961–1990, so not ‘preindustrial’) and the used definition of future climate (2070-2100)].

This warming is accompanied by a strong decrease in daily average rainfall (3-4 mm/day under RCP4.5 and a decrease of 6 mm/day under RCP8.5*), and an increase in dry spells, as illustrated in the top image.

[*) That’s a large decrease, even for the tropics. According to FAO figures we looked up, daily average rainfall in the region is between 4.7 mm/day for Belize and 8.0 mm/day for Costa Rica and Panama (the region becomes wetter moving towards the equator).]

The below graph shows (a) how the ‘NPP’, the difference between the amount of vegetation CO2 absorption during photosynthesis and vegetation CO2 emissions during respiration, decreases as the temperature goes up, indicating a decline in both general biomass and soil moisture, as shown in (b). In both scenarios a possible vegetation-promoting effect of CO2 fertilisation [that has a paradoxical CO2 emissions promoting effect on Panamanian rainforest we recall from an earlier study] is offset by decreasing soil moisture, leading to a net biomass decline:

As a result also the total Central American rainforest cover is likely to decrease considerably under the influence of 21^st century climate change – with a biome shift to either grassland or savanna climate suitability.

Central-American rainforests disappearing from the North

The authors note that although their study indicates a general decrease of rainfall under expected climatic warming, uncertainties remain, for instance regarding the response of clouds. This uncertainty would increase towards the southern part of the region, consisting of the countries of Costa Rica and Panama.

This southern part of Central America is more complicated. Even though daily average rainfall may decrease (just like elsewhere in the region) the risk of droughts in southern Central America could actually go down. The authors write the following:

“[…] regional variations are evident in both scenarios, especially over the southern part of Mesoamerica, where decreasing dry spell duration is projected along both Pacific and Atlantic coastlines.”

This could be in line with a ‘seasonally lingering ITCZ’ North of the equator (as a result of relatively rapid boreal warming) and studies suggesting the north-western part of the Amazon region is relatively resilient to climate change-induced droughts, more specifically the country of Colombia – which borders the Panama Isthmus.

In the northern and central parts of Central America the outlook for rainforest preservation is worse, especially in the RCP8.5 scenario, that shows almost a complete disappearance of climatic conditions suited for rainforest in the countries of Guatemala, El Salvador, Belize, Honduras, Nicaragua and southern Mexico.

Climate change is never the single threat

What goes for any tropical rainforest though, is that sadly climate change is never a single ecological stressor – and almost always accompanied by deforestation due to agricultural expansion. Central America is no exception, including the southern part, and as we’ve seen in part 16 of our series, at least in the case of the rainforests of Costa Rica climate change and agriculture seem to have a synergistic effect on the decline of rainforest biodiversity.

© Rolf Schuttenhelm | www.bitsofscience.org