In our this biodiversity series, we look at how to assess the impact of biodiversity changes. We looked at the basic assumptions and metrics in Part 1, and in Part 2 we looked at land use change and how that ties to biodiversity. In this article, we discuss the effect of climate change on biodiversity, and pose our final question.
In the first article, we challenged you to think about whether using biodiversity as a metric to express the impact of land transformation is as impossible as it seems, or whether you think it necessary to try, however impossible it is. After all, it is crucial that our impact assessments include biodiversity. In the second article, we described the link between biodiversity, land use and land-use change.
In this last part, we will describe the link between climate change and biodiversity, in a number of steps that describe the cause-effect mechanisms. The specific problem is that, although we know the impact historic emissions had on climate, we do not know the impact of adding or avoiding an additional kilo of emissions.
Step 1: From CO2 Equivalent To Temperature Increase
A study by Meinshausen, studying the impact of avoiding CO2 emissions, compared the so-called climate sensitivity of the current climate models, resulting in the figure below. The model has two lines: the lower line describes the immediate effect of such a reduction, and the higher line shows the impact of the equilibrium that is reached many years after the emissions have been avoided.
From this graph, we can see that every 1000 gigatonne of CO2 equivalent avoided has an effect of 2.6 degrees at equilibrium. In other words, every kilo has a temperature effect of 65e-15 °C.yr/kg, or written in full, 0.000000000000065 degrees per year.
The addition of a time dimension requires some attention. One kg does not have an impact for eternity: after about 150 years, the CO2 will be gone from the atmosphere. In this sense, CO2 emissions are similar to land use. Producing a kg of a crop only requires a certain area during certain time.
Step 2: From Temperature Increase To Biodiversity Loss
For this step, we used a Nature publication that analysed a large number of studies linking temperature increase to loss of species. From this, we could establish relationships, illustrated below with the example of butterflies. On the horizontal axis, we see the temperature increase. On the vertical axis, we see the percentage of species that will disappear due to the temperature increase.
The analysis looks at two scenarios:
- The butterflies have enough time to migrate with the change in temperature (the lower line)
- The butterflies cannot migrate with the change in temperature (the higher line), which is predicted to cause more damage
As you see, in the scenarios where it is assumed that species can migrate away, species loss is smaller.
These two illustrations show the relationship between temperature increase (horizontal) and expected loss of biodiversity (vertical)
Several of this type of studies focus on vascular plants and insects, as the impacts on higher species are more difficult to determine. If something goes wrong at the start of the food chain, most experts assume that this will affect the fate of the higher organisms.
Earlier, we described the temperature change as temporary. This also implies that emitting a kilo of CO2 only has a temporary impact on the species richness. This model assumes that when the emissions stop and the temperature decreases, the species may return. The model also treats all species as equal and does not distinguish between red-list or endangered species and other species. However, if the emission flow is constant or increases over many years, the temperature increase will also stay high, and the loss of species may be permanent.
If a species disappears from a large ecosystem, this has more impact than if it disappears from a small area. Without explaining in detail why this is, it does mean that we need to introduce an area parameter to our metric. This is consistent with what we saw in land use: to produce a crop, we need to occupy an area of land for a time.
This analysis shows why the damage to diversity can thus be expressed in terms PDF.m2.yr, which can be read as the fraction of species that disappears in a certain area during a certain time.
In the first article in this series, I discussed whether it is appropriate to express biodiversity losses in terms of the fraction of species that gets lost. In the second article, I explained how this metric can be used to capture the impacts of land occupation and land transformation. To model land use impacts, it is important to go into the fields and carefully count species. In the case of climate change, we cannot do that, as most of the damage is yet to occur. That is why we have to rely on environmental models for our impact assessment, the way we do for most impact categories.
Of course, we all know the problem with models: they try to capture a not yet fully known future. However, by using a combination of all the most widely recognised climate models, we have a relatively robust basis to calculate the temperature impact. From temperature to species loss, we partially rely on species counting, and more specifically on the observed changes that can be linked to the temperature change we already see. These observations are now being done in many parts of the world, and the data from them can be combined to determine a more robust relationship between temperature increase and biodiversity.
Again, we invite you to make up your mind: how useful do you think this modelling is for impact assessment? If this series sparked your interests, I invite you to get engaged. Follow the developments in impact assessment through the UNEP-SETAC work on impact assessment. Follow the leading methodologies, such as Impact World, ReCiPe, LIME or EPS. Or read LC-impact, a rich resource for readers.
This is the last part of the three-folded series on biodviersity, by Mark Goedkoop, originally posted at PRé Sustainability. Below you can see the first and second articles: