Climate change is a globally complex issue and there are genuine areas of uncertainty and scientific controversy. However with an increase in record temperatures, the melting of Arctic ice and a rise in CO2 there is little doubt our climate is changing.
Among the profound impacts of climate change now being investigated is its affect on ecosystems around the globe and the fact the world appears to be getting greener.
These issues, including carbon accumulation in vegetation, have been widely discussed in the scientific literature and official reports.
The Met Office’s Professor Richard Betts looks at the coverage of this in the IPCC 5th Assessment Report.
A recent paper (Zhu et al, 2016), attributed the observed worldwide greening of vegetation largely to the fertilisation effects of increased carbon dioxide. This paper was published after the IPCC 5th Assessment Report (AR5), but had the issue already been discussed in AR5? Yes – quite prominently and extensively.
The Summary for Policymakers (SPM) of the Working Group 2 volume on Impacts, Adaptation and Vulnerability features observed land ecosystem changes prominently. The first figure of results in the SPM (reproduced below) gives an overview of observed impacts of climate change, including a number of examples of terrestrial (ie: land) ecosystem changes.
The table accompanying that figure explains each of the impacts depicted on the map. Many of these clearly relate to observed greening across the world.
As well as being highlighted in the SPM, these changes and the processes behind them – including carbon dioxide fertilisation – are also discussed in several chapters of the main report. This includes the chapters on Terrestrial and Inland Water Systems (Chapter 4); Food Security and Food Production (Chapter 7); Detection and Attribution of Observed Impacts (Chapter 18); and most of the chapters on individual regions (Chapters 21-30). Some examples include:
- Terrestrial and freshwater ecosystems have sequestered about a quarter of the carbon dioxide (CO2) emitted to the atmosphere by human activities in the past 3 decades – Chapter 4, Executive Summary. There is high confidence that the factors causing the current increase in land carbon include the positive effects of rising CO2 on plant productivity, a warming climate, nitrogen deposition, and recovery from past disturbances, but low confidence regarding the relative contribution by each of these and other factors – Chapter 4, Executive Summary.
- Evidence since AR4 confirms the stimulatory effects of carbon dioxide (CO2) in most cases – Chapter 7, Executive Summary. Across all climate zones and continents, the major role of climate change and increasing atmospheric carbon dioxide (CO2) on terrestrial and freshwater ecosystems has been confirmed by new and stronger evidence – Chapter 18, Executive Summary.
- Basal area measurements at more than 150 plots across the tropics suggest that biomass and growth rates in intact tropical forests have increased in recent decades (Lewis et al., 2009). This is also confirmed for 55 temperate forest plots, with a suspected contribution of CO2 effects (McMahon et al., 2010). Satellite observations analysed in Donohue et al. (2013) suggest that an increase in vegetation cover by 11% in warm drylands (1982–2010 period) is attributable to CO2 fertilisation. – Cross-Chapter Box VW (page 158).
- Satellite observations from 1982–2010 show an 11% increase in green foliage cover in warm, arid environments (where WUE is most important) after correcting for the effects of precipitation variability (Donohue et al., 2013); gas exchange theory predicts 5 to 10% greening resulting from rising CO2 over this period – Chapter 4 section 188.8.131.52.
- Carbon dioxide (CO2) is an essential building block of the process of photosynthesis. Simply put, plants use sunlight and water to convert CO2 into energy. Higher CO2 concentrations enhance photosynthesis and growth (up to a point), and reduce the water used by the plant. This means that water remains longer in the soil or recharges rivers and aquifers. These effects are mostly beneficial; however, high CO2 also has negative effects, in addition to causing global warming. High CO2 levels cause the nitrogen content of forest vegetation to decline and can increase their chemical defences, reducing their quality as a source of food for plant-eating animals. Furthermore, rising CO2 causes ocean waters to become acidic (see FAQ 6.3), and can stimulate more intense algal blooms in lakes and reservoirs – Chapter 4, Frequently Asked Question 4.2.
In summary, there is high confidence that net terrestrial ecosystem productivity at the global scale has increased relative to the pre-industrial era. There is low confidence in attribution of these trends to climate change. Most studies speculate that rising CO2 concentrations are contributing to this trend through stimulation of photosynthesis, but there is no clear, consistent signal of a climate change contribution – Chapter 4.
The issue of land carbon uptake and increased biomass were also discussed in the Working Group 1 volume on The Physical Science Basis, both in that volume’s SPM and the chapter on Carbon and Other Biogeochemical Cycles.
Recent suggestions that this issue was downplayed in IPCC AR5 are therefore incorrect and misleading.