Ask the climate experts - your questions answered

Climate is about the statistics of the weather we experience over several years rather than the description of what the weather will be like on a particular day at a particular place.

Climate projections are a way to assess how the statistics of the weather will change in response to changes in the long-term energy balance in the climate system, caused, for example, by an increase in the concentration of greenhouse gases in the atmosphere.

In a sense these projections are more similar to a statement about a future summer being warmer than last winter (in northern hemisphere) rather than a conventional weather prediction.

Forecasting the weather is an incredibly challenging exercise. Small differences between the actual weather conditions and those used by computer models at the start of a forecast mean that the forecast and reality begin to diverge. Over the last few years very significant progress has been made and our four day forecast today is as accurate as our one day forecast was 30 years ago.

If you want to learn more the accuracy of our forecasts visit:
http://www.metoffice.gov.uk/about-us/who/accuracy/comparing

 

 

 

Ice ages have come and gone in a fairly regular cycle for the past three million years or so. There is strong evidence these are linked to regular variations in the orbit of the Earth around the sun changing the distribution of solar heating of the Earth and the seasons such that summers are cooler and winters warmer.

The last glacial period began around 116,000 years ago reaching a maximum around 20,000 years ago. The next large glaciation is relatively close on geological timescales, but that means it is expected in around 30,000 years from now. Sustained high levels of atmospheric carbon dioxide in the atmosphere that lead to a complete melting of the northern hemisphere ice sheets over Greenland would further delay the onset of the next glacial period, possibly by as much as 125,000 years.

For more information: www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-6-1.html
 

This is an interesting idea and white roofs and roads are currently being explored as one possible way in which we could adapt our towns and cities to help alleviate some of the potential impacts of climate change.

 

Indeed many cities in hot climates such as in the Mediterranean and North Africa have favoured light coloured building materials for centuries. However, despite rapid urbanisation, buildings account for only a tiny fraction of the available global land surface. Therefore, painting roofs white would not be enough to counteract the effects of global warming. 

White roofs may help mitigate climate change where they are used to reduce the need for summer air conditioning.  This would reduce energy use and associated greenhouse gas emissions from buildings but only where the summer energy saving outweigh any additional heating costs in winter. 

To learn more about how cities may respond to climate change visit: http://www.c40cities.org/

 

 

Although its concentration varies from place to place, it is safe to assume that carbon dioxide (CO₂) is a well mixed gas (http://photojournal.jpl.nasa.gov/catalog/PIA12339). This is because the time it takes for a molecule of carbon dioxide to be dissolved in the ocean, or absorbed by vegetation, is significantly longer than the time it takes the wind to displace the same molecule far away. This is not true for all gases. Water vapour, for example, is constantly evaporated and rained out, and therefore is not well mixed in our atmosphere.

One of the longest and most widely known carbon dioxide datasets is the Manua Loa time series. This can be accessed here: http://cdiac.ornl.gov/trends/co2/graphics/Mauna_Loa_CO2.jpg.

These are by no means the only observations available; CO₂ concentrations have been measured by several groups using different techniques in different locations across the planet. If you want to learn more visit: http://cdiac.ornl.gov/trends/co2/contents.htm.

An interesting comparison between local observations of CO₂ and satellite estimates is available here: http://www.youtube.com/watch?v=6-bhzGvB8Lo.

 

 

Two main points underpin scientists’ claims that man made drivers are the main cause of global warming:

1) “Carbon dioxide, methane and other greenhouse gases absorb infrared radiation and re-emit at the local temperature.” This is a falsifiable statement and if it were shown to be untrue would imply a complete rethinking not only of climate science but also of fundamental physics, astronomy and many other areas of science. Assuming this statement is true, and no other factors are at work, then a simple calculation using the observed man-made increase in greenhouse gases shows that this well understood process explains most of the recent warming.

2) The warming effect due to greenhouse gases may be amplified or suppressed by the response of the climate system (so called feedbacks). In addition there are other natural forcing factors such as changes in the amount of energy from the sun reaching the earth, volcanic eruptions, etc. Climate scientists use physical models to understand the interactions between these various factors. The models use well tested and indeed falsifiable pieces of science. Some aspects of the models can, and indeed are, falsified all the time. This helps scientists evolve their models and expand their explanatory power.

If for example, other forcing factors were shown to have a larger warming effect than previously thought, this would affect the proportion of the recent warming which could be attributed to man made drivers.

 

Since the pioneering work of the British Scientist John Tyndall in 1859 we know that not only CO₂ but also water vapour (the invisible gaseous form of water) is an essential part of the greenhouse effect.

 

Water vapour is the most effective greenhouse gas. However, it is rising CO₂ levels, not water vapour, that is driving the change in climate. Human activities are directly affecting CO₂ levels. Water vapour levels, however, are not directly affected by human activities, but indirectly as a result of a warmer atmosphere (due to CO₂ rises) being able to hold more moisture.

 

So, if we address the CO₂ increases, we also address water vapour increases. It is for this reason that CO₂ is the focus of attention. In the course of the 20th century we learned that other gases such as ozone (O3), methane (CH4), Nitrous oxide (N₂O), and more exotic gases such chloroflourocarbons (CFCs) play an important role as well. Climate scientists have looked at these gases in detail and their effects are taken into consideration in the climate predictions.

 

For more information: www.ipcc.ch/publications_and_data/ar4/wg1/en/figure-2-20.html
 

 

It would be more appropriate to consider the emissions made since the mid-20th century on total atmospheric concentrations of carbon dioxide (CO₂), rather than focus on the 1970s specifically. Concentrations of CO₂ in the atmosphere since pre-industrial times (since about 1750) have increased from approximately 280 parts per million (ppm) to around 315 ppm when routine monitoring began at Mauna Loa in Hawaii in the late 1950s, and is close to 390 ppm today. This is higher than at any point in the last 800,000 years based on data from ice-cores.

The Earth’s climate responds to the total accumulation of CO₂, and other greenhouse gases, in the atmosphere over long periods of time. The rate of increase in CO₂ concentrations has certainly accelerated since the 1970s, but climate change is a consequence of the cumulative rise in atmospheric CO₂ since pre-industrial times.

CO₂ is of particular concern as a greenhouse gas because it stays in the air for a very long time (averaging around 100 years) before being removed, and even if we stopped emitting any CO₂ today the planet would continue to warm for a number of decades due to the large inertia in the climate system.

For more info on monitoring CO₂ changes see:
http://scrippsco2.ucsd.edu/program_history/keeling_curve_lessons.html

and the impact of CO₂ emissions on climate:
www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2-1.html

Although urban areas are growing, they still constitute only a tiny fraction of the available land surface of the planet. The raw materials used to make concrete and the other materials used to build our towns and cities are already here on Earth, so the existence of the built environment is not putting the Earth out of balance.

However, the greenhouse gas emissions associated with creating these building materials do have an impact on climate. Also, world cities account for approximately two-thirds of world energy demand and the associated emissions of greenhouse gases.

To learn more about cities and climate change visit http://www.c40cities.org/climatechange.jsp

 

 

The additional energy trapped within the Earth's climate system as a result of the increased concentration of carbon dioxide since pre-industrial times is estimated to be 1.66 joules per second (Watts) per square metre of the Earth's surface, and larger if other greenhouse gases such as methane are included.

In comparison, the direct heat released from energy use is only about 0.03 Watts per square metre if averaged across the whole globe. Therefore, the greenhouse gas emissions from burning of fossil fuels have a far greater impact on the global climate than the direct heat release. However, this waste heat is very important on a local scale, particularly within larger cities where it makes a significant contribution to the urban heat island effect in which cities are observed to be warmer than their surroundings.

To learn more about this visit:
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2-1.html

The Gulf stream (or North Atlantic Drift, to give it its proper title) brings warmer water from the tropics to the North East Atlantic, giving North West Europe a milder climate than it would otherwise have. Any cooling of North West Europe arising from a slow down of the ocean circulation would be more than offset by global warming.

 

A hypothetical shut-down of the ocean circulation might see temperatures in the UK dropping by 3-5oC with night time minimum temperatures dropping by as much as 10-20oC. However, this is a ‘what-if’ scenario and considered to be unlikely. The available evidence suggests that a reduction in the North Atlantic circulation would be expected through the 21st century but no studies have been able to provide strong evidence that the circulation would shut-down.

 

However we do not fully understand the reasons for the stability of the ocean circulation, and therefore research continues to understand and quantify the risk of this potentially high-impact, but low probability, outcome of climate change.

 

For more information: www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-10-2.html

 

Projections of climate change in the UK suggest that all areas of the UK will experience warmer mean temperatures in both summer and winter, with the greatest changes occurring in parts of Southern England during the summer months.

For more information on UK climate change visit http://ukclimateprojections.defra.gov.uk.

 

 

It is true that the 3 major global temperature monitoring centres, (NASA, NOAA, and the Met Office) rank the warmest years slightly differently: NASA has 2010 as the warmest, NOAA has it as the joint warmest alongside 2005, and the Met Office ranks it as second warmest, after 1998. All 3 centres use different methodologies for the estimate of global temperature.

 

Temperatures are not monitored everywhere at all times, so there is always some level of uncertainty. But despite this the estimates are very close to each other (and all lie well within the Met Office's uncertainty range) which gives us confidence in the estimates.

Although the Met Office estimate still has 1998 marginally ahead of 2010 as the warmest, it should be remembered that 1998 was a very strong El Nino year, a natural event that tends to significantly raise global temperature. It is almost certain that 2010 is one of the 10 warmest years on record with a global average near surface temperature anomaly (relative to the 1961-1990 average) of 0.50 ± 0.09°C.

For more information: www.metoffice.gov.uk/hadobs/hadcrut3/diagnostics/comparison.html
 

 

 

There is large natural year-to-year variability in climate and this is more pronounced on a local and regional scale, than it is on a global scale. Which means that it is only possible to detect climate change over much longer timescales of at least 50 to 100 years. So when looking at recent events climate variability could –at least partially - explain the unusual observations.

 

The unusual conditions in the North Atlantic for the last couple of winters for example have played a role in the cold winters affecting Europe. Although the last 3 winters have been colder than we have been used to in recent years, they were still not record breaking winters (even allowing for the record breaking cold December in 2010). How heatwaves and cold extremes will change in the future is an area of very active on-going research.

 

For more information: www.metoffice.gov.uk/news/in-depth/severe-weather-2010 and www.metoffice.gov.uk/climate/uk/actualmonthly/

 

Using data stretching back to 1850, estimates have been made of global temperature uncertainty based on two criteria: inaccuracies in the measurements themselves and the limited observational coverage. Although the uncertainties are larger in the earlier periods they are smaller than is often thought.

 

We can make reliable estimates of annual global average temperature even in the earlier part of the data record because temperature variations occur over large distances (e.g. if it was a warm year in England chances are it would be a warm year in many other parts of Europe). This means that the global average temperature can be assessed with a reasonable number of well-spaced measurements.

 

When all of these uncertainties are taken into account the observed warming over the 20th century is significantly larger than the uncertainties associated with even the earliest observations from the 1850s.

To learn more about temperature records visit:
http://www.metoffice.gov.uk/climate-change/guide/science/explained/temp-records
 

 

The warmest year globally on record is 1998 – leading some to say climate change has stopped. But 1998 was warmed by a natural event known as El Nino, which raises global temperatures. The first ten years of the 21st century (2000-2009) is the hottest decade on record.

If you want to learn more visit:
http://www.metoffice.gov.uk/news/releases/archive/2008/global-variability

For further information on short-term trends in climate please see:
http://www.metoffice.gov.uk/media/pdf/m/6/evidence.pdf