Climate science, projections and interpretation

Published: 30th March 2011 11:35Last Updated: 14th July 2016 10:53

Climate Projections

The Climate Systems Analysis Group at the University of Cape Town are the lead weADAPT organisation working on climate modelling and climate science. CSAG use a process of empirical downscaling which looks at historical records to derive a relationship between the type of large-scale weather patterns experienced and the resulting climatic conditions (e.g. rainfall and temperature) at local weather stations. This relationship is then applied to model output from global climate models and used to produce station-level climate projections.

The basics of Climate Change

This section was prepared by Mark Tadross, at the Climate Systems Analysis Group, as part of a climate change study done for Mozambique.

What is climate change?

Climate change refers to a change in the average weather experienced in a particular region or location. The change may occur over periods ranging from decades to millenia. It may affect one or more seasons (e.g. summer, winter or the whole year) and involves changes in one or more aspects of the weather e.g. rainfall, temperature or winds. Its causes may be natural (e.g. due to periodic changes in the earth's orbit, volcanoes and solar variability) or attributable to human (anthropogenic) activities e.g. increasing emissions of greenhouse gases such as CO2, land use change and/or emissions of aerosols. In contemporary society the term "Climate change" often refers to changes due to anthropogenic causes.

How is the climate changing?

It is widely recognized that there has been a detectable rise in global temperature during the last 40 years and that this rise cannot be explained unless human activities are accounted for (IPCC, 2007). The regional distribution of temperature increases is not however uniform and some regions have experienced greater change than others, especially the interior of continental regions such as southern Africa. This is consistent with detected increases in annual temperatures found over southern Africa since 1900 (Hulme et al, 2003). Additionally these changes in temperature are associated with decreases in cold extremes accompanied by increases in hot extremes (New et al, 2006). Furthermore, the global average temperature indicates an increasing rate of change, such that temperature is rising quicker during the latter half of the 20th century. Importantly, this increase in the rate of change is expected to continue, potentially resulting in more rapid changes of climate in the future.

Changes in rainfall are typically harder to detect due to its greater variability, both in time and space. Even so, changing rainfall patterns have been detected for many parts of the globe, including moderate decreases in annual rainfall over southern Africa. Where records are of sufficient length there have been detectable increases in the number of heavy rainfall events (Solomon et al, 2007) and within the southern hemisphere there is evidence for a moistening of the tropics and subtropics (Zhang et al, 2007). This is consistent with regional studies over continental southern Africa which have shown trends for an increasing length of the dry season and increases in average rainfall intensity (New et al, 2006). This has important implications for the seasonality of regional rainfall and together suggests a shorter but more intense rainfall season.

Besides changes in temperature and rainfall, other aspects of global change are notable (IPCC, 2007):

  • Increases in intensity and spatial extent of droughts since the mid-1970s;
  • Decreases in northern hemisphere snow cover;
  • Increases in the duration of heat waves during the latter half of the 20th century;
  • Shrinking of the Arctic sea ice pack since 1978;
  • Widespread shrinking of glaciers, especially mountain glaciers in the tropics;
  • Increases in upper-ocean (0-700m) heat content;
  • Increases in sea level at a rate of 1.8 mm yr-1 between 1961 and 2003, with a faster rate of 3.1 mm yr-1 between 1993 and 2003.

There is therefore compelling evidence for climate change at the global level, attributable to human activities. However, understanding how global climate change may affect individual countries and small areas within a country is still a matter of research and is inherently linked to issues of uncertainty (see below). So whilst the observed global level changes serve to highlight that climate change is a reality and that we have confidence in continuing and potentially accelerating change, it is necessary to explore how local climates may already be changing as well as how they are expected to change in the future.

What causes climate change?

Anthropogenic emissions of greenhouse gases (the main cause of anthropogenic climate change) have increased steadily since the industrial revolution. The rate of emissions, however, have been steadily increasing over time, and computer models of the earth's climate system (including both natural and human causes) are unable to simulate recent warming unless they include anthropogenic emissions of greenhouse gases. Computer models of the earth's climate which include only natural forcings (e.g. solar variability due to both internal and orbital variations, volcanic activity etc.) simulate a cooling of the earth after 1960, which is at odds with the observed warming. This has led the Intergovernmental Panel on Climate Change (IPCC) to conclude recently that most of the warming of the last 50 years is attributable to human activities.

Understanding uncertainty

The issue of uncertainty is crucial to understanding past and future climatic change, especially when designing adaptation strategies. Uncertainty does not mean that we have no confidence in our projections of future climate, it just means that the way in which the information is used needs to take appropriate account of this. The IPCC define four sources of uncertainty that currently limit the detail of the regional projections:

1. Natural variability: Due to the limiting factor of observations (both in time and space) we have a limited understanding of natural variability. It is difficult to characterise this variability and the degree to which it may exacerbate or mitigate the expected background change in climate. This variability itself may change due to anthropogenic factors, e.g. increases in the frequency of droughts and floods;

2. Future emissions: Much of future projected change, at least in terms of the magnitude of change, is dependent on how society will change its future activity and emissions of greenhouse gases. Even so, the world is already committed to a certain degree of change based on past emissions (at least another 0.6ºC warming in the global mean temperature). Human responses to managing emissions may result in a projected global mean temperature change of between 1.5º and 5.6ºC;

3. Uncertainty in the science: This is further complicated within Africa because of limited understanding of the regional dynamics of the climate of the continent. There may be aspects of the regional climate system, which could interact with globally forced changes to either exacerbate or mitigate expected change e.g. land-use change. One consequence is the possibility of rapid nonlinear change, with unforeseen and sudden increases in regional impacts;

4. Downscaling: The term used to define the development of regional scale projections of change from the global models (GCMs). Downscaling tools can introduce additional uncertainty e.g. between downscaling using regional climate models and statistical techniques. Usually this uncertainty limits the confidence in the magnitude of the projected change with the pattern and sign of change often interpreted with greater certainty.

References

  • IPCC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, New York, US, Cambridge University Press.
  • Hulme, M., R. Doherty, T. Ngara, M. New and D. Lister (2001). African Climate Change: 1900-2100. Climate Research 17(2): 145-168.
  • New, M., B. Hewitson, D. B. Stephenson, A.Tsiga, A. Kruger, A. Manhique, B. Gomez, C. A. S. Coelho, D. N. Masisi, E. Kululanga, E. Mbambalala, F. Adesina, H. Saleh, J. Kanyanga, J. Adosi, L. Bulane, L. Fortunata, M. L. Mdoka and R. Lajoie (2006). Evidence of trends in daily climate extremes over southern and west Africa. Journal of Geophysical Research 111. D14102, doi:10.1029/2005JD006289
  • Solomon, S., D. Qin, M. Manning, R. B. Alley, T. Berntsen, N. L. Bindoff, Z. C. A. Chidthaisong, J. M. Gregory, G. C. Hegerl, M. Heimann, B. Hewitson, B. J. Hoskins, F. Joos, J. Jouzel, V. Kattsov, U. Lohmann, T. Matsuno, M. Molina, N. Nicholls, J. Overpeck, G. Raga, V. Ramaswamy, J. Ren, M. Rusticucci, R. Somerville, T. F. Stocker, P. Whetton, R. A. Wood and D. Wratt (2007). Technical Summary. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. S. Solomon, D. Qin, M. Manning et al. Cambridge, UK. New York, US, Cambridge University Press
  • Zhang, X., F. W. Zwiers, G. C. Hegerl, F. H. Lambert, N. P. Gillett, S. Solomon, P. A. Stott and T. Nozawa (2007). Detection of human influence on twentieth-century precipitation trends. Nature 448: 461-465

 

Climate Science and Interpretation

For an introduction to (or a refresher of) the basics of climate science, we recommend you have a look through these resources:

Also, some notes on basic climatology, summarised from presentations given at an ACCCA Training of Trainers workshop on climate science and risk communication can be found in here. From the same workshop summaries of presentations given by Mark Tadross on climate modelling and the process of downscaling.

The majority of projections of future climate come from Global Circulation Models (GCMs), which vary in the way they model the climate system, and so produce different projections about what will occur in the future. These differences can be highly significant, for example some models may show a region getting wetter, and some would show it getting drier. A fundamental approach of the weADAPT group is to explore this uncertainty and to look at the full range of climate projections when planning adaptation, prioritizing adaptation options that are robust against a wide range of changes rather than relying on output from one model. This is why the climate change explorer uses output from multiple models. Some of the reasons for this model uncertainty are explored here and in notes from a session on uncertainty by Bruce Hewitson.

Guidance on how to interpret output from multiple models can be found here

Useful references include:

10 years of weADAPTweADAPT has been sharing knowledge on climate adaptation for the last 10 years.

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