Archives For October 2015
If we are not certain about the weather next week, how can we be confident about climate change predictions 50 or 100 years into the future? The charts below show two predictions about the temperature in Europe.. one was forecast for a day in June 2015, some 198 hours ahead, the other is a prediction for European July temperature by 2100, some 876,000 hours ahead. If computer model forecasts beyond 120 hours are known to become significantly more unreliable pushed further into the near future, how confident can we be in similar computer models predicting atmospheric conditions some 876,000 hours into the distant future?
Of course, weather and climate are different things: weather is short term atmospheric conditions over days and weeks and is usually forecast at high resolution over small areas, while climate describes long term weather over decades or more for larger regions or the whole planet. Nevertheless, much like the computer models that broadcast meteorologists use to issue short range weather forecasts, climate models use equations of fluid motion and thermodynamics to determine the behaviour of the atmosphere and ocean and to project predictions of Earth’s climate into the future. Both short range and long range predictions require powerful super-computers to run similar complex models that ingest millions of real time observations and perform trillions of calculations to produce predictions of atmospheric conditions on a huge three dimensional grid across different surfaces and altitudes, including the oceans.
The atmospheric system is essentially unpredictable. Even the most powerful super-computer short range weather predictions can become quite unreliable beyond about 120 hours. This is perhaps why the MetOffice still only widely publish short term forecasts out to 5 days. It’s also why charts beyond 300hours are sometimes called “Fantasy Island”… they are so unreliable and not to be taken seriously as a forecasting tool on their own.
In the medium range, accurate ten day forecasts are still something of a holy grail and longer range seasonal forecasts arguably remain largely experimental as they are based on the unreliable time-frame of most numerical model output and the application of complex teleconnections and /or the extrapolation of observations from historic analogue patterns. The MetOffice have even put their seasonal long range predictions into obscure parts of their website after notable public failures in past seasonal forecasts, where they still languish today despite huge investment in computer power. The charts below show examples of MetOffice contingency planner long range forecast information.
Even with a “perfect model”, weather and climate prediction will always suffer from uncertainties due to the immense complexity of the climate system, the chaotic nature of the atmosphere and the simplifications and approximations necessarily built into models themselves. The ensemble charts below illustrate this growing uncertainty over just a short time scale of a few weeks. Note the increasingly wide range of possible outcomes from the individual members showing the growing uncertainty as time progresses, and this is in a relatively settled period of weather.
So, if we really cannot be not certain about the weather next week, how can we be confident about related computer model predictions of climate for 50 or 100 years ahead? To answer this we need to outline the three different types of uncertainty over climate change prediction:
There are three main types of uncertainty over climate change predictions:
- Model uncertainty: climate models have to approximate and estimate feedbacks and processes, they do this slightly differently.
- Internal variability: weather is chaotic partly due to uncertain internal forcings, like volcanic eruptions.
- Scenario uncertainty: future estimates of human behaviour and emissions of greenhouse gases in particular are uncertain
So, do these uncertainties increase over longer time scales thus rendering computer model predictions of climate in 100 years completely unreliable? The answer is “No”! Or more precisely, most of these uncertainties actually decrease over time making model predictions of climate in 100 years possibly more reliable than a seasonal forecast for next summer! Time scale and geographic scale are two reasons that can explain why this happens:
- Time scale: climate models deal with longer timescales better than short. The chart below shows lines indicating different model output for temperature change between 1850 and 2010. Note the various models (shown as different lines) all reaching a similar overall temperature increase of +0.8C by 2010.
Over the long timescale shown above, the model runs all agree on an overall temperature rise of +0.8C due to initial changes in radiative forcing linked to combinations of, for example, increases in emissions of greenhouse gases, changes in solar radiation and frequency of volcanic eruptions etc. Models confidently handle these changes over long time scales because climate system and model uncertainty both decrease over time..
Now… if we zoom into one part of the chart above the predictive capability of the model is shown to be much more questionable as the lines wiggle about much more over shorter timescales and sometimes go in completely opposing directions. This shows uncertainty increasing over shorter timescales.
On the shorter timescale of decades or less, model uncertainty increases as climate variability over short time scales is naturally large from one year to the next i.e. one year can be colder or warmer than the previous due to internal variability of the climate system (i.e.chaos). Models don’t handle this small scale short term climate chaos very well! One model run predicts cooling in a particular decade, while another predicts warming for the same decade. It turns out that predicting climate change over smaller timescales is more unpredictable than predicting changes over broad sweeps of time! This is because of the internal chaotic nature of the climate system and model uncertainty being greater at this higher resolution. For example, models will handle these short term variables differently: How will ocean heat uptake respond? What will happen to ocean currents and regional climates? How will snow cover respond? Will volcanoes erupt? How will cloud cover and type change? … and many more besides.
Despite these transient climatic uncertainties over short time scales the overall direction of change towards a new climatic equilibrium in the long term is confidently predicted by all the models. So… longer timescales are better handled by models than short.
The exception to this reduction in uncertainty over time is scenario uncertainty. Most long range climate model charts include a wide range of predictions. This wide range is not due to model uncertainty or uncertainties over internal climate variability. Scenario uncertainty is the uncertainty over predicting future emissions of greenhouse gases. In other words, uncertainty over our own human behaviour in the future, which is historically difficult to predict! This is the reason why the IPCC charts show several “RCP trends”. Representative concentration pathways (or emissions scenarios) show a range of human response to the climate crisis… ranging from drastic curbs to emissions and lower growth, through business as usual, to increased emission pathways presumably due to high growth with no curbs to greenhouse gas emissions. These all yield obviously different outcomes. The uncertainty over human response, known as the scenario uncertainty, increases over time. Despite scenario uncertainty, each RCP is an accurately modelled temperature change based on changes in radiative forcing, the wide range of results is due to our own unpredictable behaviour more than climate chaos or uncertainties in the models.
2. Geographic coverage
On a long term global scale, climate system uncertainties are reduced while uncertainties increase over how climate change will occur over small geographic regions. Predicted global mean annual temperature change is therefore more certain than, for example, regional European or UK temperature change in 100 years. Future climate change on regional scales is more uncertain than on a global scale due to small scale variabilities.
So… prediction of climate change over short time scales and regional scales is more uncertain than predicting longer range changes on a global scale. Models struggle to resolve climate change at small time scales and small geographic scales. Overall however, longer term climate outcomes are more certain than short term small scale weather action. Even with the immense complexity of the climate system, computer models can provide confident predictions of future climate within a range of scenarios. The IPCC quote levels of confidence and certainty for their climate predictions and, even for the end of the century, they claim high confidence in their predictions ranging from “likely” to “more likely than not” for various scenarios.
In conclusion, maybe this analogy might help: Perhaps climate prediction is a bit like a football match: complex uncertainties about precisely where the ball will go during the game are perhaps completely impossible to forecast beyond the first few seconds of the game. Any detailed minute-by-minute action on the field thereafter becomes increasingly uncertain over time as countless variables come into play, including some chaos. For example, individual player performance on the day, how the players and teams interact, the chaotic nature of the ball, the nature of the pitch… these are internal variables that make it almost impossible to model accurate step by step action of a game perhaps beyond the first few kicks after the whistle blows. This is why small scale high resolution weather forecasts are still limited to less than a week ahead. Nevertheless, despite such short term uncertainty the overall score and outcome of the game is still possible to predict with confidence, particularly within a certain range. The same can be said for long range climate models.
This post has only scratched the surface of climate and weather uncertainty. Please let me know of any errors you spot in this post. Further reading available here:
Reigate September summary weather statistics
Average temperature 13.3C
Total rain 79.8mm (CoCoRaHs)
Max gust 25mph (14 Sept)
Mean SLP 1016mb
Sunshine 132.9 hours
September in Reigate turned out to be cooler than average with rainfall amounting to around average totals mostly falling in showers so that there were relatively long dry sunny spells, especially towards the start of the month, with HIGH pressure in charge or nearby for much of the time.
It was sunnier than usual for September at over 130 hours. This was good news for Run Reigate which turned out to be a beautiful day for the runners.
Much of the September rainfall fell in heavy showers, especially in some storms mid-month. This meant 2015 September rainfall at nearly 79.8mm turned out wetter than 2014, at a mere 22mm. Despite this it was still just about equal to the long-term average rainfall according to MetOffice anomaly charts.
At a mean temperature of 13.4C September was slightly cooler than average and the CET came out at 1C below the long term average. Globally, September was 0.31C warmer than the long term average.
Hurricane Joaquin, previously a Category 4 hurricane, tracked across Bermuda today and dropped to Cat 1. Nevertheless, maximum sustained winds were near 85 mph with higher gusts. Joaquin is expected to transition to a large extra-tropical low pressure system on Wednesday. The chart below shows how many extra-tropical storms (those named previously as tropical cyclones) reach or have got near to the UK, many of these would have passed unnoticed as regular autumnal stormy weather.
The tracks below show the remnants of Joaquin apparently taking direct aim at the UK but it’s not unusual to have extra-tropical storms arriving in UK waters during the autumn. Of course, extra-tropical storms lose much of their tropical characteristics and potency as they recurve and track across the cooler mid-latitude Atlantic. Even if extra-tropical storms track directly across the UK they are usually not much more than a nuisance storm, like Bertha in August 2014.
Beyond Friday, model tracks are more uncertain on whether the remnants of Joaquin will actually impact the UK at all, or linger offshore. A ridge of high pressure builds later this week and acts as an effective buffer to block off Atlantic LOWS, including Joaquin. He is therefore reduced to sitting offshore.
Of the main models the ECM sees ex-Joaquin taking a more direct track across the country later in the weekend. The GFS is keen to disrupt the trough and send a secondary LOW across the south of the UK later in the weekend while the main system seems to merge with a trough to the NW. The UKMET sticks the LOW offshore for longer and sweeps the system to the NW of the UK with any wind impact mostly limited to the far west. This disagreement between models shows uncertainty so … keep watching forecasts if weekend weather (especially later) is important.
Prior to this the pressure across the country is set to build after an unsettled start to the week. The charts below show a purposeful rise in pressure to the end of the week, with the Joaquin uncertainty kicking in thereafter, although Saturday looks good on most charts at present.
The Atlantic satellite view shows the impressive swirl of Hurricane Joaquin tracking near Bermuda today and the UK top right.