Tillmann Mohr, a special advisor to the secretary general of the World Meteorological Organisation, talks to Science, People & Politics.
Interview by Helen Gavaghan
"Weather is nothing other than physics and chemistry of the atmosphere. Climate is the average of the weather over a given location over a 30 year timespan. This definition has existed for 100 years."
Thank you for agreeing to this interview with Science, People & Politics. As a special advisor to the secretary general of the World Meteorological Organisation you have one of the most important jobs globally today in the field of weather forecasting and climate change.
Science, People & Politics:
Do you have any doubt that human activity is contributing to a global warming of the Earth's climate?
Tillman Mohr:
I have no doubt. Human activity is causing global warming. It is incontrovertible.
Let me give you two reasons for my certainty. First, measurements of temperature taken all around the world during the past 100 years show an overall increase in the average global temperature of 1 degrees centigrade. There are regional variations within that, of course, and sometimes the temperature increase slows down for a short period, but without doubt in the past 100 years measurements already taken show that the average temperature globally is one degree centigrade higher today than it was in 1910.
I should also say that this one degree increase in the global average temperature manifests itself as much higher over the Arctic than over the oceans, where one sees in some places a decrease. There are simulations for every country.
Secondly, if one runs a computer simulation starting 100 years ago of what would be happening now to the average global temperature if there were no human beings on Earth then we see a decrease in average global temperature - global cooling - during the last 50 years.
That is, if one considers only natural phenomena such as the precession of the planet's orbit around the Sun, the precession in the planet's orientation, its tilt, toward the Sun, changes in the Earth's magnetic poles, variations in the Sun's output and its known physical cycles then we are at a natural physical stage where, over the past 50 years, there would have been a global cooling not the warming we have seen. Putting these two things together is what makes me say that the observed warming of the average global temperature over the past 100 years is directly attributable to human activity (anthroprogenic warming). We are seeing an average global temperature increase despite the fact that naturally, and without human beings on Earth, there would have been an average global cooling over the last 50 years in the same period.
It is true there has been no increase in the measured global average temperature this past 10 years but that does not change that there has been an increase during the past 100 years when without human beings one would expect cooling over the last 50 years within this 100 year period. It is also true that climate change caused by human activity is not new. For example, deforestation in Medieval times led to local and regional climate changes. What is different now is that human activity is impacting changes on a global scale. Ozone in the stratosphere and the ozone hole over Antarctica are a prime example of the global impact of human activity on climate. Human activity has put more chlorofluorocarbons into the atmosphere, and the physics and chemistry of the atmosphere have carried those to the stratosphere and resulted in an ozone hole over Antarctica.
The world of meteorology has known since the 1960s that global climate changes were because of human beings. It was clear something was wrong but it was not clear what, and the first World Climate Conference was held in Geneva 1979. An outcome was establishment of the so called Global Climate Observing System (GCOS), which provides the long time series of climate data necessary to detect climate changes. The next World Climate Conference was held in 1991 and it established the International Panel on Climate Change (IPCC). The depletion of stratospheric ozone was one of the first indication of human caused global climate change.
Science, People & Politics:
The French National Weather Service reports on its website that the global temperature rise will be between 1.1 and 6.4 degrees centigrade by the end of the century. What do you think of this prediction?
Tillman Mohr:
Put simply it depends on the scenarios that were used in the simulations. The French website is reporting predictions for 2100 that depend on carbon dioxide concentrations in the atmosphere. There are models for carbon dioxide from 390 to 1100 parts per million (ppm). Associated with those concentrations there are predicted increases in global average temperature. Many other factors are also at play because carbon dioxide is not the only gas contributing to global warming as a result of human activity. Nor are atmospheric gases the only variable impacting global change. Aerosol are also very important.
Science, People & Politics:
Is the decrease in temperature over some parts of the Oceans the reason why climate change modellers are so interested in the way the oceans and atmosphere interact and want to see models developed that "couple" ocean and atmosphere?
Tillmann Mohr:
Yes. The atmosphere interacts with the oceans and with the land surfaces. So you need coupled models of the three components to get realistic results of the changing climate.
Science, People & Politics:
Are you saying that if, globally, the increase were 3 per cent by 2100 then the way the temperature increases will vary regionally, and thus the way the climate changes will also vary regionally? So climate change might be very different on the Gold Coast where ocean impacts the region, compared with Chad in the centre of Africa. And is it that the average temperature increase globally will depend on human activity from farming to how many cars are on the road to what fuel we use to make electricity and thus how many chemicals we put into the atmosphere that trap heat on Earth and contribute to a global warming?
Tillmann Mohr:
Yes, to all of those questions.
Science, People & Politics:
How damaging to mitigation strategies, sometimes for decades into the future, are the controversies that have surrounded the quality of data and data sets related to climate change? What questions ought government departments and local authorities (Federal and Cantons, for example) to be asking about the data on which they are making short, medium and long term planning decisions.
Tillmann Mohr:
The criticisms are unfair. There are mistakes in the IPCC reports but there are hundreds of pages of report. And the errors do not undermine the overall conclusion that human activity has caused global warming.
I have heard people criticize the quality of the data collected during the past 100 years that show there has been an observed increase in average global temperature over the past 100 years. This average is constructed from ground-based measurements at meteorological stations all over the globe, and from ocean buoys and ship measurements. People say they have been in places where the instruments are not of good quality. That is true, but this global average is based on data taken from many meteorological stations established by member states of the World Meteorological Organisation (WMO - a UN body) and its predecessor the International Meteorological Organisation. Often instruments are from the same instrument maker, or are made to a particular specification and the instruments can be callibrated against an external measure which is the same for all the instruments.
Although the WMO's secretariat has no policing method of its own for verifying the quality of instrumentation and observations its larger more established member states do work in co-operation with those with less developed national meteorological services to provide validation services for the quality of data sets. And when we talk of the average global temperature we are speaking of averages constructed from thousands and thousands and thousands of measurements, so many and taken so frequently from so many unrelated locations that it is possible to make some mistakes of human error or for there to be faulty instrumentation but for those few incorrect readings to be swamped by the data of unarguably good quality. In the context of the global temperature average the measurement from one thermometer, say, has little meaning.
Human activity is, without doubt, causing global warming.
Science, People & Politics:
You are the WMO secretary general's special advisor on a global framework for climate services. What is the WMO wanting to do?
Tillmann Mohr:
Unlike Europe and other developing*(I think the previous word probably ought to have been developed, rather than developing. Insert by editor 22.10.11.) countries most developing countries do not have a well advanced climate sector within their national meteorological services. Other than the UK and France, which started as weather prediction services, most of Europe's meteorological services started out as climate services. Africa and south America have the fewest and least developed climate services. The WMO wants to see that situation change. The five categories of work within the global framework on climate services are: observation and monitoring; research and modelling; climate services; climate mitigation; and capacity building. By the way, though the climate services vary considerably all the WMO's member states have access to the same quality-controlled weather data.
Science, People & Politics:
How would you define weather and climate? For example when I think of weather I think of Thomas Jefferson recording temperatures and pressures. Whilst when I think of climate I think of it as being the typical atmospheric conditions for a particular geographical location. For example, the UK is damp and the Sahara is dry. Would you mind taking a shot at a more erudite and sophisticated explanation of weather and climate?
Tillmann Mohr:
Weather is nothing other than physics and chemistry of the atmosphere. Climate is the average of the weather over a given locations over a 30-year timespan. This is a definition that has existed for 100 years.
Science, People & Politics:
I understand that all these satellite data - millions and millions of pieces of data twice a day - fit into computer simulations of the weather. So if instrumentation on an ocean buoy records an atmospheric temperature of 10 degrees c at 12.00 GMT on 10th March 2010 in the Bay of Biscay then all over the world there are computers containing grids of co-ordinates - like three dimensional graphs - that this data will be sent to. Some of the grids cover a three dimensional box above the whole northern hemisphere or Europe or just the atmosphere above France, Spain, Portugal and Britain, say. These computers will already have gone through a pretend scenario -- a best guess of what the weather will be like tomorrow, the day after the magazine publishes this interview. So the temperature measured by the buoy tomorrow can be compared with the temperature the computer will have guessed will be tomorrow's temperature. Is that right? Is that how numerical simulation works, guessing from an understanding of weather patterns how the weather will develop and then correcting the simulation so the the simulation starts its next future guess from a more accurate picture of reality?
Tillmann Mohr:
Not quite. Let me clarify. Take a weather forecast model for two weeks rather than a climate model. At some stage in the beginning real data of pressure, temperature, humidity, wind direction and speed were analysed and with this analysis the forecast run started over a particular time period, say 6 hours. The numerical forecast model runs a set of equations, which describe the atmosphere and the surfaces the atmosphere is interacting with. After, say, 10 minutes of computer time, which might be a prediction of the weather 6 hours ahead, one can ask the computer what its predictions are. These results are checked against reality based on new observations. And a new forecast run is starting. This is repeated until the desired forecast period, say two weeks, is achieved.
Science, People & Politics:
What are the most important parameters to measure for weather forecasting and at what scales of time, temperature and radiation frequency and what are the most important parameters for monitoring climate? How has understanding this past 20 years deepened of which parameters are the most significant for weather forecasting and, separately, for climate monitoring?
Tillmann Mohr:
The physical measurements needed are the same for climate and weather. But for climate one needs to be much more accurate (0.1 rather than one degree centigrade) because of the length of time over which one is averaging. The first World Climate Conference called in 1979 by the WMO established GCOS and the observational requirements were identified as Essential Climate Variables (ECVs). These were the variables that needed to be taken globally to assess the nature of climate and climate change. Within the set of ECVs there are 26 ECVs which can be derived from satellite measurements. These variables set by the GCOS might be calculated from data measured by R&D or operational satellites. Atmospheric ECVs include parameters such as the surface wind speed over the oceans, recordable via active scatterometers detecting the reflected radiation from the dynamic surface of oceans. Other ECVs (and these all need to be cross checked in many different ways) include vertical soundings for temperature, pressure and water vapour. This adds up to four or five million vertical soundings taken over the day. Only now do we have the computers to digest so much data.
Science, People & Politics:
You also advise the WMO's secretary general on satellite matters. How have satellites improved data collection for weather forcasting?
Tillmann Mohr:
Satellites have reduced considerably the errors in numerical weather prediction. Their importance for the quality of weather forecasts is today far beyond those of the conventional data like the measurement of the upper air soundings by ballons. Three satellite instruments at present in orbit called active microwave sounding units AMSU (see links below to a description of the instrument) have contributed alone to an 18 per cent reduction in the errors in numerical weather predictions.
Science, People & Politics:
What part did you play in establishing the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT)?
Tillmann Mohr:
I was a member of the German delegation. German foreign policy in the mid 80s was to increase the number of European international organisations in Germany. The number in Germany of such bodies was low compared with Switzerland, France, Austria and the UK. For example the WMO is based in Geneva and the European Centre for Medium Range Weather Forecasting is in England. In addition to Federal policy the town of Darmstadt in Southern Germany and the Rhein region also had an economic interest in attracting another European intergovernmental organisation to Germany, as did the German Weather Service. The European Space Agency invited those countries which had expressed an interest in forming EUMETSAT to participate in an intergovernmental conference in June 1986. Our delegation was headed by Dr. Heinz Reiser, the director of the German Weather Service. We prepared papers looking at the importance for the economy, foreign relationships and the stimulus EUMETSAT could provide for high tech. industry in the local economy. There were only a handful of people in the organisation when EUMETSAT was formed first in 1986. Now there are 500 people on staff or as contractors.
I was invited to be part of the delegation because I had been very involved at the European Space Agency (ESA) with Europe's first geostationary satellite programme - Meteosat. ESA had an interest as a research and development body in not running operational programmes. And Roy Gibson, then ESA's first director general, was supportive of the establishment of EUMETSAT. Regis Tessier within ESA was very supportive. ESA also established EUTELSAT in the commercial sector and tried with navigation but failed. You have told this story in your History of EUMETSAT written for EUMETSAT.
Science, People & Politics:
What would you say meteorology is and what have the changes been in the way that the field is studied since you were first a student at the Universities of Wurzburg and then Frankfurt?
Tillmann Mohr:
Both numerical weather prediction and satellites have become enormously important. When I was doing my Ph.D satellites were new. I studied satellite imagery for Typhoon Dinah in 1967 and my thesis in 1970 proposed a mechanism for the way that tropical storms become extra tropical cyclones. At the time of my Ph.D three mechanism were thought to be possible. I concluded from study of satellite imagery that a weakening of the vertical convection plus strengthening of horizontal winds disrupts the tropical storm structure but that when Typhoon Dinah reached the tropical latitudes to the North, where the Coriolis forces are stronger, the storm reformed as a cyclone. The general conditions to be met, I concluded, needed to be a surface pressure low and a fast moving low travelling horizontally in the upper atmosphere. The wind vector fields combine to shape a cyclone. My work had predictive value for cyclone strengths and paths starting with satellite imagery for a few years and then was superseded by numerical weather prediction.
Science, People & Politics:
How did you become a meteorologist, what attracted you to the field? Did you realise at the time what a thoroughly international field of study it is? One which simply cannot be undertaken effectively unless one views the Earth as a whole and without artificial boundaries such as State lines.
Tillmann Mohr:
It was very simple. I went to a lecture by a professor who I thought was interesting. I had no thought of meteorology at the time, but he made both me and my friend laugh -- at him a little bit -- and then he interested us in what he was saying.
Further information available from the following sites. Links disabled by editor, 22.10.11.
The French National Weather service.
The German Weather Service.
The World Meteorological Organisation.
The European Organisation for The Exploitation of Meteorological Satellites.
The European Space Agency.
NASA links to articles about active microwave sounding units (I found these on a Harvard site).
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