Note: This is the second post reviewing an article in Science magazine’s special section called ‘Natural Systems in Changing Climates’ that covered a range of contemporary research about the impact of recent climate behavior on ecosystems and species stability. This post will be discussing the article “Changes in Ecologically Critical Terrestrial Climate Conditions’ found here.
As climate change begins to affect surface temperatures and local weather patterns more drastically, plant and animal species must respond to their changing climate in two ways to survive: 1) migrate to a new geographical location that matches the same climate as their original habitat’s climate, or 2) adapt with new behaviors or evolve to allow survival in the new climatic conditions. Unfortunately, as we will see below, the anthropogenic cause of the current temperature increases and weather extremes may be occurring at too rapid a pace for the species to respond. Therefore, it is crucial for scientists to predict which regions will be most affected by climate change and manage risks for species that will need our help in adapting to the new future that is quickly approaching.
The majority of our collective knowledge about the effects of climate change on ecosystems arises from scientists delving into the geologic record. Researchers match the fossil distributions of plants and animals from past eras with the same animal’s present geographic distribution; any shift in locations suggested climate change that caused the species to migrate to a new location that matched their original habitat’s climate.
But this information is not everything. The Anthropocene era (beginning around ~8000 years ago when humans became sedentary and began farming) has resulted in a very different type of climate change due to human causes, mainly in terms of speed, and therefore fossil records may paint a very different picture compared to what we should expect in the future. The key tool for researchers to explore this uncertain future is climate modeling. These models usually take the form of computer simulations containing vast quantities of data about spatial distributions of climate variables such as surface and ocean temperature, greenhouse gas emissions, land use, etc. In particular, researchers are hoping to use climate models to predict rates of climate change in various regions and the speed at which different species can migrate.
The goals of these studies are to 1) understand which parts of climate change biological systems are most sensitive to; 2) compare current climate change with past to see if we can use past migration patterns to understand future patterns; and 3) understand the context in which climate change is occurring so we can identify constraints and opportunities on ecosystem preservation (e.g., human land usage).
May the Forcing Be With You
To understand how these models work, first we must understand an important term in the climate modeling community: forcing. When discussing climate models, you will often hear terms such as anthropogenic forcing, radiative forcing, CO2 forcing, and many more. What does this mean? Forcing variables come in as external parameters that we set to see how their input changes the model dynamics. These are known as initial conditions because the forcing variable is set as a certain value at the beginning as an input to the model. We can always change this constant value and then rerun the model to see how ranges of values change outcomes. Researchers will often choose a forcing variable so they can see directly how that variable affects outcomes like temperature or weather. It is a way to experimentally test the direct effect of changing that one variable on everything else in the system.
The rest of the variables, known sometimes as internal variables, are included in the model along with a set of interactions based on mathematical equations. Based on the combination of the equations and the forcing variables, these internal variables interact in nonlinear ways that create feedback mechanisms between all the variables, leading to very different outcomes depending on initial conditions. Remember, the interactions we input are just our best educated representation of how nature actually operates! So we can always be improving models by updating the interactions in a way that seems more accurate based on new data we’ve gathered. It’s an organic, iterative process to fine-tune these models.
So, for example, if we want to see the effect of CO2 concentration on temperature, we will run the same model with all the same internal variables and interactions, but changing the amount of CO2 concentration (our forcing variable) over a certain range. For each value, we’ll get a new spatial distribution of temperature across the entire globe. The beauty of this is that, from all this data, we can find certain thresholds of CO2 concentration, which are the key levels above and below which the model takes very different paths. This drastically different trajectory that occurs on opposite sides of a threshold value is very common incomplicated, nonlinear systems such as the Earth’s climate system.
How Do We Model
Climate depends on three main factors, generally categorized as past, present, and future information:
1) Energy imbalance due to GHG already built up in atmosphere (past). When we say energy imbalance, we mean that more energy is being absorbed by the earth than being emitted to outer space. The sun emits radiation that hits Earth and is the source for essentially all energy available to us (although it changes forms!). The Earth and atmosphere both absorb this radiation and then emit their own, at a different wavelength. However, the emitted radiation from Earth is partially absorbed by the atmosphere blocking its path to space. More greenhouse gases in the atmosphere means that it absorbs more of the radiation from Earth. The atmosphere then re-emits this radiation, partially to outer space, partially back to the Earth. It is this re-radiation back to the Earth that is the main source of surface temperature increases. That’s global warming in a nutshell.
2) Sensitivity of current climate to change due to human-induced factors (anthropogenic forcing) such as the carbon cycle that is influenced by fossil fuel emissions.
3) Impact of future emissions of greenhouse gases and aerosols on climage change rates.
For the first two, we use geologic records, but the future is uncertain, so we use models and input different parameters to understand where certain thresholds lie for certain future alternatives. These models are nonlinear, which means they are very sensitive to boundaries and initial conditions as discussed above.
These models are generated using data from 25 differnet modeling centers (known as the Coupled Model Intercomparison Project, or CMIP5). Since we do not know the exact amount of emissions to come in the next century, we can then plug in different values of radiative forcing into the model to see our possible futures. Radiative forcing is the measure of the difference in energy per time per area (W/m2) radiated back to Earth compared to that escaping to outer space. This value increases with greenhouse gas emissions, so it’s a way to represent future scenarios of varying amounts of emission. In this case, they input a range of forcing from 2.6 W/m2 to 8.5 W/m2 – 2.6 is an optimistic estimate that will become reality only if we work hard to decrease emissions compared to current standards, and 8.5 will be the reality if we do nothing to lower emissions. 2.6 would require global economy-wide emission standards that lead to a net removal of CO2 from the atmosphere. The article focuses on this 8.5 W/m2 input as this is the likely scenario if we do not work to lower emissions.
OK , so what do the models show us? The results are shown here:
Focus on the middle left and bottom left figures, as these show results from the 25 modeling centers discussed above. The scale at the bottom is in units of temperature difference compared to the mean temperature between 1986-2005. Put most simply, by 2046-2065 (mid 21st), all land regions become warmer by at least 2 C across the entire globe. Warming is greatest in the high latitudes of the Northern hemisphere with 4-5 C increases in surface temperature. By the end of the century (late 21st, bottom left), all land regions see 5 C increases and the Northern hemisphere reaches 6 C increase.
So what does this mean? Well, that’s a story for a whole different blog post. But there is a great National Geographic documentary about how the world will change if temperatures increase by 6 C – I definitely recommend a watch: http://www.youtube.com/watch?v=TKo4TSq40l0.
But the main point is an increase in extremes: by around 2080, these models predict more than 80% of land regions on the planet will have mean summer temperatures above the maximum temperature ever recorded over the comparison time period (1986-2005). This increase in temperature corresponds with increasingly dry climates near the Equator, with regions in Central America South America, and Africa predicted to have less than 30% of the minimum precipitation recorded in the late 20th century.
These extremes do not only occur over seasons, but also within daily timescales. Daily maximum temperatures will be increased across the globe and the number of frosts and cold days decrease, which are often crucial for stopping the wildly expansive nature of many insects we consider pests. And, as we may already being see the beginning of, these extremes lead to higher rates of tornadoes, hurricanes, and severe thunderstorms.
Inertia in several very different arenas means that no matter what actions we take, we will continue to see climate change and temperature rise into the mid 21st century. These include: 1) ocean thermal inertia due to its enormous heat capacity; 2) inertia of current fossil-fuel economy; 3) inertia of increasing population with increased energy demands; 4) human rights inertia to improve global well-being will lead to electricity needs and increased energy demands; and 5) political inertia. It is the fate of the late 21st century that we still have a chance to change by reducing greenhouse gas emissions, but this will require a great push against the political inertia and creative tactics to introducing ways of meeting the energy demands of the increasing population without using fossil fuels.
Can Ecosystems Keep Up?
One more type of data is important for considering ecosystem survival: climate change rate, which includes rate of change of temperature, rate of extreme weather incidence increases, etc. If we look at the past, all cooling and warming cycles occurred on MUCH longer timescales, on the order of millions of years. In the present, we’re looking at time scales of decades! This means we have no idea if species can respond quickly enough to current/future rates to survive. Evolution occurs on time scales much longer than this, which seems to indicate that the only possibility for species is behavioral adaptation or migration.
Climate change velocity – distance per time that species need to move to keep same climatic conditions – is the central measure that researchers are now using to understand the effects of changes on ecosystems. Models suggest 1 km/year over the 21st century, but below you can see the spread across the globe given an 8.5 W/m2 radiative forcing by the end of the 21st centurty:
The graph shows a color scale of climate change velocity in units of km/year. Notice regions indicating over 100 km/year that are near oceans and mountains? These constraints will limit where species can go, and in these cases, extinction may be likely.
This is a burgeoning area of research that is still being developed, but many important factors are involved. Will species reach a point where the same conditions cannot be found, due to human or non-human barriers (e.g., oceans and mountains!) or novel combinations of temperature and precipitation arise? Scientists are working hard in this new area to understand which species must be attended to. This is important to study in combination with traditional climate models to emphasize ecosystem preservation!
This review article demonstrates the severe temperature increases that climate models predict, up to 6 C, if greenhouse gas emissions continue as they are. This is just another clear result showing that it should be one of the top international priorities to cut greenhouse gas emissions. This will likely cause capitalistic discomfort in the short-term and slower economic growth as we adjust to considering the costs of greenhouse gas emissions (which we should have been doing all along!), but the long-term gain cannot be described in terms of dollar amounts. It is about preserving (to the extent we still can) the climate we need to thrive on this rocky home, about preserving those species with whom we cohabit this planet and rely upon for subsistence. We must keep close tabs on this rate of climate change and how it will affect species movements, so that we may accommodate them and their needs in these drastically changing times.
Diffenbaugh NS, & Field CB (2013). Changes in ecologically critical terrestrial climate conditions. Science (New York, N.Y.), 341 (6145), 486-92 PMID: 23908225