By Jennifer Hushaw
The discussion of global precipitation will be covered in two parts. This bulletin (part one) provides an overview of the precipitation trends observed over the last century, as well as projections of future change. Part two will cover a range of forest ecosystem and management implications of changing precipitation patterns and drought stress.
Observations of Global Precipitation
Observations suggest that there has been an increase in globally averaged precipitation on land over the last century. This is based on several global datasets that all show a statistically significant increase since 1900 (Figure 1). However, the datasets do not agree on how much it has increased. This primarily stems from a lack of sufficient data across the globe, especially during the early 1900’s. The upward trend is also much clearer in the early part of the record than in the years after 1950. As a result, there is only low confidence in the observed global change prior to 1950 and medium confidence in the trends since that time.
The data also reveal notable variability in precipitation trends at different latitudes, but problems with data quality, coverage, and agreement have resulted in low confidence in most regional observations. However, there is at least one area of the world where clear trends have emerged – namely, the mid-latitudes of the Northern Hemisphere (30⁰N to 60⁰N). Here, the data show a likely overall increase in precipitation, with medium confidence prior to 1950 and high confidence afterward (Figure 2).
Future Global Precipitation
There are several metrics to consider when thinking about changes in future precipitation, these include changes in the total annual amount, shifts in timing, and changes in the intensity of individual precipitation events – all of which have important ramifications for soil moisture dynamics and water availability for forests.
On average, models project a gradual increase in global precipitation over the 21st century. This is because warmer temperatures drive more evaporation and the increase in water vapor leads to more precipitation. While we expect an increase in the total global amount, this does not mean every individual region will receive more precipitation. Instead, some areas will actually become much drier because we also expect existing regional precipitation profiles to be amplified – a wet-get-wetter and dry-get-drier pattern. In particular, it is very likely we will see an increase in precipitation at high and some mid-latitudes, whereas in the subtropics an overall decrease is more likely than not (Figure 3).
Note: In the near-term, these projected changes are likely to be fairly small compared to natural variability from circulation patterns like the El Nino-Southern Oscillation, for example. This will also be true for some regions even in the long-term under a low emissions scenario (see extensive hatching in low emissions map, above). This dominance of internal variability at certain scales is an important recurrent theme.
Shifts in the timing and/or amount of precipitation from one season to another can significantly affect ecosystem processes and the hydrologic cycle. Models suggest that we will see these types of shifts and the largest changes will be at high latitudes. In particular, the contrast between the wet and dry season is likely to increase in most places as temperatures warm. Although, there will be some notable regional exceptions along the equator and poleward edges of the subtropical dry zone, where changes in atmospheric circulation will cause precipitation patterns to shift completely. In both mid- and high-latitude regions the projected increase is larger for winter than summer (Figure 4). However, this increase in average winter precipitation will occur at the same time that increasing temperatures contract the length of the snow season.
Warmer temperatures allow the air to hold more water vapor when it is saturated, so when it rains or snows in a warmer world there is simply more water coming down, which will increase the intensity of individual precipitation events. As a result, the probability of heavy precipitation is projected to increase, and the effect will be stronger with increased warming (see Fischer and Knutti 2015, Figure 3 (a), (b), and (c) for maps showing projected change in the probability of heavy precipitation events under different levels of warming across the globe; units are probability ratios, i.e. the ratio of current or future probability to the pre-industrial probability). As a greater fraction of total rainfall comes in the form of these heavy downpours it will also affect soil moisture because more precipitation will be lost as runoff, rather than being absorbed by the soil or going to replenish ground water supplies. For these reasons, intense rainfall events will also increase the likelihood of flash floods.
Uncertainty of Precipitation Projections
We have a high level of confidence in estimates of future temperature because those projections are based on basic physical principles and significant model consensus, but precipitation is much more uncertain. And, as with temperature, the uncertainty grows as we scale down – from estimates of change in the global average to regional projections. There is less agreement among global climate models regarding the direction and magnitude of precipitation change, especially in certain regions.
There are a number of reasons for this uncertainty, including:
- The small spatial scales on which precipitation dynamics take place (e.g. cloud microphysics)
The scale of these processes is much smaller than the resolution of global climate models, so those dynamics cannot be simulated from first principles, instead they must be described using parameterization, which introduces some room for error.
- A large degree of spatial variability from one location to the next
More data coverage is necessary to accurately capture precipitation trends, in contrast with temperature, which is highly autocorrelated over long spatial scales (i.e. the temperature at one location is closely related to the temperature nearby, so just a few data points can be used to calculate a large area).
- The indirect relationship between greenhouse gases and precipitation change
At a very basic level, greenhouse gases influence the climate by changing the amount of heat, which changes atmospheric pressure, which leads to changes in rainfall patterns. Because the influence of greenhouse gases is several steps removed, it is more challenging and complex to simulate how changes in those concentrations will affect precipitation. However, in recent years scientists have improved their understanding of the mechanics underlying projected changes in precipitation and, as a result, we have a much higher degree of confidence in some of the patterns that have emerged from global climate models (e.g. wet-get-wetter, dry-get-drier).
Drought: A Closer Look
Changes in the length and severity of drought are important metrics of precipitation change. Generally, drought implies a moisture deficit relative to some previous norm. More specifically there are three types of drought: meteorological, hydrological, and soil moisture (Figure 5). Of these, soil moisture drought is perhaps the most relevant for forests because it is closely tied to plant water availability.
Soil moisture drought is driven by reduced precipitation and/or increased evapotranspiration. In fact, at seasonal or longer time scales, an increase in evapotranspiration (which is indirectly driven by temperature) can lead to more frequent and intense periods of soil moisture drought. Soil characteristics, rooting depth, vegetation, an on-going lack of precipitation, and previous soil moisture and groundwater conditions also play an important role in determining this type of drought risk.
The 2013 Intergovernmental Panel on Climate Change improved previous work on soil moisture projections by using a consistent soil depth across all climate models in their report. The results yielded high confidence in regions where surface soils are expected to dry, but little-to-no confidence in locations where surface soils are projected to be wetter. The models also consistently projected drying conditions in certain regions, specifically the Mediterranean, northeast and southwest South America, southern Africa, and the southwestern U.S. (Figure 6). Although, they disagreed on the direction of change (wetter or drier) in other large regions, such as central Asia or the high northern latitudes. Consequently, there are only a handful of locations where we can currently be confident about how soil moisture will change in the future and those locations are all projected to become drier.
Part two of the global precipitation bulletin will delve into how and where these projected changes in precipitation, drought, and soil moisture are likely to impact forests. It will also include a discussion of the optimal species characteristics, silvicultural decisions, and infrastructure design for minimizing risk from these climate variables and capitalizing on growth potential.
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