Forests, in their role as massive carbon stores

Besides being an integral part to the climate system, Forests (especially thick tropical rainforests) are a large storage of organic carbon. The carbon is taken up by terrestrial “sinks” through plant growth/primary productivity and stored within their structures, as well as in the soils underneath. Looking at the world scale, out of all carbon stored in terrestrial ecosystems, about 55% is stored in tropical forests which is at a density of 242 MG C per hectare, most of which is stored in the biomass. Lands are said to become net emitters when ecological disturbances or deforestation cause a reduction in primary productivity. Deforestation or land-cover conversion to less productive ecosystem will result in the emission of CO2 to the atmosphere via forest burning (prominent in the Amazon in the land conversion to pasture), decomposition of harvested matter, fuel-wood burning and/or soil respiration (IPCC.www).

Here I want to present a map that shows in overview the carbon sinks and sources (Pg C per year) over two time periods arising from global forests - and I will focus on South America. 
(Downward facing bars indicate storage, upward facing indicate emission)

Source: click here

We can see that the South American continent plays a large role in terrestrial carbon fluxes. What I found interesting here is that in the period 2000-2007 the net flux is still a sink but at a lower value compared to the previous period 1990-1999 (-23%) despite reduced emissions from gross deforestation. And to understand this, my topic of land-cover change is highly important. Deforestation emits CO2 almost directly as described above, but it doesn't stop there: the area left as pasture (for example) starts to act as a source again as the grasses grow, but at a much lower productivity. Therefore, the fact that the South American total area of intact tropical forest is reduced over time is in itself a loss in 'potential carbon sink' and contributed to weaken the continents total "sink" characteristic.

It is estimated that about half of anthropogenic CO2 emissions are taken up by natural sinks like the oceans and terrestrial ecosystems, but this hasn't been stable through time. A widely cited paper by Schimel et al (2001) shows that terrestrial systems have only been a net sink since 1990, powered by land use changes that supported regrowth and nutrient fertilization that enhanced primary productivity. Can these changes power a further increase in carbon uptake in the future? I quickly became aware through my reading that scientific opinions and modelling studies on the future of forests as carbon sinks is highly divided and uncertain. The straight-forward conceptualisation of "more forest- more carbon stored" is not completely accurate. It surely depends on the health of the forest (relates to my posts on forest degradation and edge effects), climate-vegetation feedbacks, and ecological changes that occur within the forests in response to climate pressure (e.g. Amazon drought 2005 shows that this is NOT negligible by turning Amazon from sink to a source through the loss of biomass (Philipps et al 2009)).

A conceptual model linking causes and effects of disturbance/change in the Amazon basin

I had read the article "The Amazon basin in transition" a while back - and now that my blogging is soon coming to an end in a couple of weeks, I think it is useful to post this graphic produced by Davidson et al (2012). It shows the interactions between global climate, land use, fire, hydrology, ecology and human dimensions within the Amazon basin.

Please have a look and see how my blog posts fit to (or are controversial to!) this conceptual net of linkages: 

Burning biomass and emitting aerosols in smoke - what are the links to climate parameters?

Biomass burning, whether to fuel settlements (as discussed in post 17/11/15) or as the cause of slash-and-burn land clearing within the Amazon, elevates atmospheric aerosol levels (besides of course emitting CO2...). The map below shows the "smoke aerosol distribution (D < 2.5 μm; in μg m–2) and wind field in the BL over South America during the transect flights from Rondonia to the western Amazon" (Andeae et al, 2004).

These increased local aerosols (aerosol optical depth) within the Amazon basin in biomass-burning season, have shown to be correlated to increased cloud cover, cloud height and increased rainfall by Lin et al (2006). 

Another study, by Koren et al (2004), concluded from satellite imagery that heavy smoke over the Amazon basin reduces cumulus cloud formation by up to 37% compared to zero-smoke. 

Andeae et al (2004) has a different take at examining the "Smoking of rain clouds" - they show that cloud droplet size is reduced, delaying precipitation rain-out at lower levels. This means, that the aerosols and particulates from the biomass burning can travel longer distances in the upper cloud formations and thus spread the effect over larger areas. What I found interesting here, is that they suggest that with this delayed precipitation, cloud cover increased in height (consistent with Lin et al 2006) and tops may overshoot into the stratosphere. With the "smoke" and water vapour now becoming part of stratosphere, these very locally sourced pollutants can have an impact on large scale circulation patterns, and especially partially affect the radiative properties of the Earth. Due to the complexity of these mechanisms and factors involved, the extent of this effect cannot yet be accurately modelled, but the probability to influence the global climate circulations has been suggested to be high.

As important as this mechanism may be, Roberts et al (2003) analysed this too but concluded that the dominant factor influencing a change to the hydrological system in the Amazon remains the dominant land-use change from forest to pasture.

Thoughts: 

We can see that there are different conclusions drawn from the examination of how the smoke of burning biomass in the Amazon affects the local and even global climate parameters. One thing is for certain - the affect on cloud cover (whether it is the increased height, reduced droplet size or even reduced/increased cloud-cover all together) makes up a forcing factor that alters the natural climate. With the alterations to the natural climate induced by the patterns of land-cover change, this added forcing is another complication for the ecosystems and populations to react to. 

Altered micro climate conditions increasing convective rainfall above cleared land in the transition zone to natural forest cover

One phenomenon, which I actually observed myself when I travelled to Yucatan in Mexico a few years ago, was that more clouds seemed to form above the deforested (pasture) land than above the natural forest adjacent to it. In this post, I want to explain this local climate effect and how this impacts the ground.

Building on what I have written about in earlier posts on the local climate effects of deforestation, we now know that temperatures tend to increase over deforested vs. natural forest land-cover. We also understand that there is much greater moisture recycling over forest area, due to higher rates of evapotranspiration, deep cumulus cloud formation and subsequent rainfall. The contrast that exists here, between the micro-climate characteristics of the two land-covers side-to-side, produces a sharp gradient in the landscape.

Especially the temperature differences are responsible for causing an effect often termed a “vegetation breeze”. These breezes enhance convection circulations above the deforested area on the transition to the forest, but draw moisture away and reduce cloud-cover over the forest itself. 

Knox et al (2010) used an analysis of past satellite imagery and forest-cover data to show the impact of the “vegetation breezes” on rain-bearing cloud formation. Findings generally concluded that, as expected, cloud cover is decreases in deforested areas far off from natural forests, but a particular increase of rain-clouds are shown on the non-forest side of the transition zone (within+/-10km). 

Cloud cover over Alta Floresta in Brazil, taken by  NASA satellite imagery, showing that cloud cover can be greater above the cleared land than the adjacent natural forest. (Source: click here)

The impact of this phenomenon on local and meso-sclae precipitation was further analysed in a large modelling study by Garcia-Carreras and Parker (2011). Varying heat-exchanges and amount of cleared/forested land-covers (PlanetEarth, www), they found a clear coupling between land heterogeneity and convective locally-generated rainfall. Rainfall increases 4-6 fold on a transition zone, compared to one continuous land-cover. Also – and this is quite important when thinking about the “edge effects” of deforestation – rainfall is suppressed over natural forest next to this transition zone to a cleared area. The degree of the suppression is related to the degree of the climatic gradient formed between the land-covers. 

Further thoughts on this:

When putting this phenomenon into the context of my previous posts on the wider climatic effects of deforestation in South America, we see that these small-scale effects may enhance those larger patterns. In particular, in the regions (e.g. the southern and north-eastern Amazon) where a drying signal is already threatening rainforest resilience, this further suppression of rainfall may quicken forest loss by inducing negative feedback. 

The findings of Garcia-Carreras and Parker (2011) also suggest that different deforestation practises and patterns (e.g. large clear-cut vs. selective or fish-bone deforestation) will change the impact of the suppression signal – which should be taken into account by forest management. 

The edge-effects (e.g. this impact on locally generated rainfall) impairs the "buffering effect" of rainforests to climatic variability, extending far into the natural forest. The highly fragmented land-cover seen in South America means that a lot of the remaining forest does no longer remain in stable condition, even if left untouched. Ewers and Banks-Leite (2013) back this up, by showing that about 12% of Atlantic Brazilian forest is impacted by such altered micro climate characteristics.

Increased convective rainfall over the cleared land can have different effects on the ground, ranging from benefiting farmers with greater moisture supply for crops to increasing surface runoff and erosion of soils. 

Further on: Global climate effects of tropical South American deforestation

Following one of my reader’s comments, I decided to look at the impact of South American deforestation on the  global climate. A very recent review in ‘nature climate change’ by Lawrence and Vandecar (2015) is very informative on this issue, and I have presented one of their figures in my last post.

Global Circulation Models incorporate the hydrological cycle, and represent vegetative canopies with their effect on energy and water. “The atmosphere and biosphere form a coupled system whereby climate influences vegetation distribution and ecosystem function, which feed-back to affect climate” (Lawrence and Vandecar, 2015). Studies therefore use these to examine potential impacts of wide-scale land cover change on the global climate system.

The hypothetical situation of removing all tropical forests is analysed in several studies. The predicted increase in global mean temperatures ranges from 0.1–0.7 °C (of which the highest value is equivalent to doubling the warming cause by all GHG emissions since 1850!). This occurs as the cooling effect, which is a cause of the high rate of evapotranspiration in tropical forests, is lost through deforestation. Generally, the increase in temperatures is still greatest in the pantropical areas (due to the effects discussed in post 03/12/15). 

Global mean precipitation is predicted to be left unchanged – however, Lawrence and Vandecar (2015) explain this to be primarily due to the opposing signals of local and regional impacts.

What I find interesting are the extra-tropical impacts of regional deforestation. The link that exists between regions of deforestation and regions of impact is a teleconnection (and arises by altering, for example, geopotential height, Rossby waves, Ferell and Hadley cell circulations).

Regional deforestation in the South American tropics can have severe effects on the climate over remote regions via teleconnections. I want to give some examples of this. Avissar and Werth (2005) show that tropical deforestation of the Amazon and tropical Africa have severe far-reaching impacts on regional precipitation in the U.S. Midwest. Here, precipitation is reduced dramatically especially in seasons in which it is most important to support local agricultural productivity. Gedney and Valdes (2000) predict Rossby wave propagation due to Amazon deforestation, directly affecting the winter climate over the Northern Atlantic and Europe with increasing rainfall. 

We must not forget that a complete deforestation of all tropical forests is unlikely to happen, but assumed in many of the impact studies. Medvigy et al (2011) compare this with a ‘business as usual’ (incremental) deforestation of the Amazon. This new decadal-scale, meso-scale resolution modelling approach shows that precipitation changes are less than previously predicted in GCM models. While the general regional effects of deforestation are maintained in the ‘business as usual’ scenario, the degree of impact is much smaller. The extratropical impacts under full-deforestation scenarios (e.g. examples given in last paragraph) did not occur under the incremental deforestation scenario. 

Lawrence and Vandecar (2015) examined various GCM studies that incorporated various levels of deforestation and suggest that “tropical forest clearing beyond ~30–50% may constitute a critical threshold for Amazonia”, beyond which the impacts on local climate (and ecosystem functioning) and remote climate will be dramatic.

Concluding thoughts:
Reviewing different model simulations, differing in their resolution and area/degree of deforestation, shows that the exact impacts of deforestation cannot yet be predicted with certainty, as these depend on the exact occurrence of land cover change and the nature of the teleconnections with other parts of the world. What we may conclude, however, is that the potential of climate alteration by deforestation far off the deforested areas is great. This finding supports the argument that deforestation is a global problem.

Remote regional impacts of tropical deforestation on precipitation.

To introduce my upcoming blog post, I wanted to give a little snippet in advance: this highly informative map, summarising the remote regional climate impacts of tropical deforestation. Here we look at changes induced to precipitation patterns.

Map shows: Projected increases (circles) and decreases (triangles) in rainfall in full-deforestation scenario of the Amazon (red), southeast Asia (blue) or central Africa (yellow).  Source: Lawrence and Vandecar (2015)

It gives us a first indication of how far-reaching the effects of complete tropical deforestation will be. Amazonian deforestation will likely decrease precipitation in much of the U.S.'s Mid-West, the Caribbean and parts of South-East Asia. Increases are expected over northern Europe, eastern North America and East-Africa. What I found interesting in evaluating the predicted cumulative impact of tropical deforestation is that there will be little net change to global mean precipitation. 

An astonishing collection of satellite imagery to show 21st century forest cover change - a focus on South America

Research by Hansen, Potapov and Moore et al (2013) was built on newly available high-resolution satellite data. Interestingly the U.S government held this data previously but did not allow it to be used in research - despite the the commonly recognised threats to the worlds forests and associated ecosystem services... (click for source)
What came to attention is the intense forest loss in South America: but not necessarily in the highly researched Brazilian Amazon! Evidence from Paraguay and Bolivia, for example, showed that the highest forest loss has been occurring in the subtropical forests of the world.

This GIF image shows the drastic change 2000-2012 in Paraguay, but please also pay attention to the zoom on the whole South American continent. Highlighted in red are areas that were converted by anthropogenic processes - it will shock you and justify the relevance of this blog further.

(click for source)

The scale of land cover change impact on the climate system: a complex matter

The paper by Hoffmann and Jackson (2000) explores how the conversion of tropical savannah to grassland influences climate and finds a similar pattern of climate evolution to what I have outlined in my previous post (concerning the conversion of tropical forest to grassland). I want to take this, slightly different example, to explore the complexity revolving around scale and location of impact.
Despite a common pattern of change (increased temperature, lower seasonal rainfall, lower ET), the study shows strong regional variations in the climatic effect of an assumed universal conversion of land cover.

Within each of the world regions examined, some areas existed where precipitation is expected to decline by considerably more than 10% (see the dark red shading in Figure 1). While the authors point to the stochastic nature of the model used as a potential reason for this prediction, it very likely represents the real variability to be expected. Notably, with uneven land cover change over a tropical biome (as realistically not ALL of the land will be converted) it is even less predictable where the effects will hit most strongly, posing challenges to risk management.

Figure 1: click for source

Another complicating factor to the location of climatic impact is that through deforestation, roughness length of the vegetation is decreased, having significant effects on circulation patterns and thus making the rainfall pattern in an area more uncertain.

What this paper touches upon too, but Pires and Costa (2013) examine in more detail, is the concurrent land cover changes happening on the South American continent, with accumulating and interacting effects on the climate and biomes. They show that inner Amazon forest regions keep their rainforest biome characteristics, but outer forest regions may cross a forest-savanna bioclimatic threshold even at low deforestation levels. Different regions vary in their resilience to be pushed to the next “climate-vegetation equilibrium state” (Oyama and Nobre, 2003). Due to the climate-vegetation feedbacks induced by the warming, drying climate response to deforestation locally and as spread effects of adjacent land degradation in the cerrado savannah, the bioclimatic boundary between rainforest and cerrado is predicted to move 500-1000km north into the Amazon basin. Hirota et al (2011) have a more methodological perspective on how to model the climate effects of land cover change – but highlight just as effectively that deforestation in the tropics in particular may have local, meso and even global scale effects, which interact with the climatic effects of other region’s land cover change. Thus, observed and also modelled climatic changes must be evaluated upon the imminent local drivers, as well as on the relative impacts of more far-reaching changes.

Brazil disappoints with increased deforestation rate just before Paris Climate Change Conference

In the news: theguardian
While deforestation figures had been reducing steadily in Brazil since the implementation of new forest conservation policies, just before the COP21 in Paris, official deforestation figures of the past year August 2014-15 were published that meant a major "setback" for Brazil. The rate of deforestation had increased by 16%. Brazilian environment minister Izabella Teixeira acknowledged the let-down and blamed the increased pressure on land-cover conversion to feed growing livestock-company demand.