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Met Office scientists protecting our forests from pests and pathogens



Today is the United Nations International Day of Forests, and in this blog post we explore the importance of this work.

The Climate and Plant Biosecurity Climate Service, funded by the Department for Environment, Food and Rural Affairs (Defra), is a collaboration between the Met Office’s Vegetation-Climate Interactions team, Defra’s Plant Health Risk and Horizon Scanning team, the University of Exeter, Fera Science, the University of Warwick, Forest Research and The Royal Botanic Gardens, Kew.

Oak processionary moth caterpillars. This species is a non-native pest of woodlands. Picture: Adobe Stock.

Since 2006 the non-native and invasive Oak Processionary Moth has been spreading across England and Wales. Picture: Adobe Stock

The aim of the service is to provide analyses, tools and guidance to help manage the climate-related risks to UK plants, particularly trees and forests, from plant pests and pathogens.

Pest and pathogen outbreaks

Pests and pathogens present serious risks to our trees and forest habitats as well as the ecosystem services they provide. The number of new pest and pathogen outbreaks affecting trees has increased rapidly in recent years (see table, Source: Forestry Commission).

Year (since 1971)             New tree pest or pathogen outbreak

  • 1971      Dutch elm disease
  • 1983      Great spruce bark beetle
  • 1984      Phytophthora alni
  • 1995      Gypsy moth
  • 1997      Dothistroma needle blight
  • 2002      Phytophthora ramorum
  • 2002      Horse chestnut leaf miner
  • 2003      Phytophthora kernoviae
  • 2005      Bleeding canker of horse chestnut
  • 2006      Oak processionary moth
  • 2006      Phytophthora pseudosyringae
  • 2007      Pine tree lappet moth
  • 2010      Acute oak decline
  • 2010      Phytophthora lateralis
  • 2012      Ash dieback
  • 2012      Asian longhorn beetle
  • 2012      Sweet chestnut blight
  • 2012      Phytophthora austrocedri
  • 2014      Phytophthora sikiyouensis
  • 2014      Sirococcus tsugae
  • 2015      Oriental chestnut gall wasp
  • 2017      Elm zigzag sawfly
  • 2018      Eight toothed spruce bark beetle
  • 2021      Phytophthora pluvialis

Climate (variability and change) influences pests and pathogens in many ways, including i) the timing of life cycle events (such as emergence from egg to caterpillar), ii) the spatial distribution and spread, and iii) the introduction and establishment of non-native species.

Tools to help manage UK plant biosecurity

The UK Climate-Pest Risk Web Tool is one of the tools that has been developed by Met Office scientists Neil Kaye and Deborah Hemming (Vegetation-Climate Interactions team) in collaboration with biosecurity and forestry experts at Defra, Forest Research, Fera Science and the University of Warwick. It integrates ecological knowledge and models of known temperature thresholds for different pests/pathogens, with up-to-date climate observation datasets from the Met Office National Climate Information Centre.

Deborah Hemming, Scientific Manager of the Vegetation-Climate Interactions team at the Met Office, who leads this climate service, notes: “When tree pests and diseases become established, they can wreak havoc on our woodlands. Commercial forestry can be affected hugely, but they also affect the landscapes of our islands which many people love and cherish. In the 1970s, Dutch Elm Disease killed most of the UK’s stately elm trees with those plants remaining being small stands in isolated sections of hedgerow. Similarly, Ash Dieback since 2012 has decimated ash trees with similar devastating effect.

“When trees die, especially native broadleaf trees, there are impacts on landscape and wildlife. And in times of climate change there is also a reduction in the availability of carbon stocks because trees provide a hugely valuable ecosystem service by drawing down atmospheric carbon and locking it away. By joining forces with experts in plant biosecurity at Defra, ecological modelling at University of Exeter, forestry at Forest Research and ecological systems at Royal Botanic Gardens, Kew, we are able to provide scientifically robust and useful research, tools and services to help protect UK trees and forests now and into the future.”

The tool enables users to easily estimate the timings and locations of pest outbreaks across the UK, and inform actions to assess, survey, monitor and eradicate plant pests, helping to enhance UK plant biosecurity.

Simon Toomer, Curator of Living Collections at Royal Botanic Gardens, Kew, says: “As we develop strategies and plans to adapt and prepare our tree and shrub collections for changing climatic conditions, one of the most complex and least understood threats is that from pests and diseases. This research is going to help us understand how changes in general climate variables translate into changes in the specific conditions experienced by pest species, and how we may adapt our management accordingly.”

Healthy ash trees form an important part of the UK's tree canopy. Isolated trees are also a feature of hedgerows. Picture: Adobe Stock

Healthy ash trees form an important part of the UK’s tree canopy. Isolated trees are also a feature of hedgerows. Picture: Adobe Stock

Improving monitoring and modelling of microclimates

Pests and pathogens respond to microclimates within the habitats where they live. To improve the estimates of pest/pathogen risks, ecological modellers at the University of Exeter have developed mechanistic microclimate models to estimate temperature and humidity within relevant habitat locations e.g., under tree canopies, inside tree trunks or buried at various depths within the soil. In these habitats, microclimates can vary by 40-50°C and be significantly different to conditions observed at weather stations.

Ilya Maclean, Professor of Global Change Biology at the University of Exeter, says: “It is important to understand the climate as pests and pathogens experience it. This can be very different from the conditions measured by a weather station. My team is developing models that allow us to do this and the data we are collecting as part of this project will be invaluable in helping us improve our models”.

Daegan Inward, Senior Scientist at Forest Research, explains further: “We know that beetle outbreaks are often associated with sun-warmed stems, and understanding the under-bark microclimate is important to help predict the risk of insect establishment and population growth in Britain.”

Climate change and globalisation

To validate the microclimate models, in February 2024 the Met Office, the University of Exeter, Forest Research and Kew Gardens began a campaign of microclimate monitoring at five sites in different forest habitats across southern England (Cornwall, Dartmoor, Alice Holt, Kew Gardens and Wakehurst). The team installed a series of sensors to monitor temperature within the woodlands and inside tree trunks. The data collected will help improve the understanding and modelling of variations in the pest/pathogen microclimates, and therefore the estimates of biosecurity risks.

Defra Chief Plant Health Officer Nicola Spence said: “Climate change and globalisation are increasing the number and diversity of pests and pathogens we are exposed to, resulting in an ever-growing number of threats. This collaborative effort to develop climate modelling tools and improve our understanding of pest or pathogen climate interactions, will allow us to better plan for and improve our ongoing surveillance and monitoring. This work is crucial to adapt to a changing climate and better understand how the health and resilience of our trees could be at risk.”

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NEMO: a numerical ocean model



A numerical ocean model is a computer programme representing the equations of motion (momentum, conservation of mass and thermodynamics) for the ocean. The model stores each of the physical properties of the ocean (temperatures, salinities and currents) on a three-dimensional grid, writes Ana Aguiar.

Ocean models store physical  properties such as salinity, temperature and currents on a three-dimensional grid. Picture: Adobe Stock

Smaller ocean features can be resolved by using a finer grid with more points, but this requires more computational power. The model evolves these physical properties forward in time using its equations of motion. Models of sea ice and biogeochemistry work using similar principles.

Why do we need a numerical ocean model?

We need these models to predict the state of the ocean within short and long timescales for a variety of purposes, ranging from support to operations at sea (for example, search and rescue) to understanding the role of the ocean in the Earth’s climate system. As the ocean sits beneath the atmosphere, sea-surface temperature patterns have widespread impact on the weather over land. Largely because two-thirds of the Earth is covered by ocean and the heat capacity of water considerably outweighs that of the air, the ocean acts as a regulator of the atmosphere.

In polar regions temperatures become cold enough for seawater to freeze and sea ice forms on the surface of the ocean. Sea ice plays an important role in the climate system because it insulates the ocean from the colder atmosphere in winter and, being whiter than the ocean, reflects sunlight in the summer.

The NEMO modelling framework includes a sea-ice model component, known as SI³ (Sea Ice modelling Integrated Initiative). The sea-ice component is run along with the ocean component in a similar manner but using a different set of equations. To understand and prepare for climate change we need to account for the role of the ocean and sea ice.

How is the NEMO model developed?

Nucleus for European Modelling of the Ocean (NEMO) is a state-of-the-art ocean modelling framework. NEMO is developed by a European consortium with the objective of ensuring long-term reliability and sustainability of the code. In other words, the task of maintaining and developing such a complex computer programme requires a well-coordinated team effort, involves tens of developers and hundreds of users.

In the UK there are two member organisations: the Met Office and the National Oceanography Centre (NOC). Met Office Scientific Manager in Ocean Modelling, Ana Aguiar explains: “We work in partnership through the Joint Marine Modelling Programme, contributing to the development of NEMO. The code is publicly available for use in research and commercial applications. It is imperative to reach as many users as possible, to ensure the code gets tested and pushed to the limits of its usability. User requirements then prompt further advances.”

NEMO benefits from continual work to improve its performance (scientific and computational efficiency), to incorporate new scientific and process understanding, and to exploit the increase in supercomputer resources. When the developments are sufficiently mature and can provide significant scientific or technical improvements, a new NEMO version is released. Along with scientific upgrades (which tend to be increasingly computationally demanding), we must deliver code optimisation to make the best use of the available computing resources.
This video presents how NEMO is used by the Copernicus Marine Environment Monitoring Service.

What’s next?

The next NEMO release (expected to be rolled out this summer) will deliver significant improvements to model performance allowing it to run considerably faster. In the long term, among other things, we are also working towards porting the NEMO code to Graphical Processing Units (GPUs) to ensure continuity of the code in future mainstream High Performance Computing architectures

During April we are exploring the topic of the ocean and climate. Follow the #GetClimateReady hashtag on X (formerly Twitter) to learn more throughout the month.

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What’s the pollen outlook this year?



The Met Office launches this year’s Pollen Forecast today (Friday 22 March) which will run until mid-September.

One in five people in the UK suffer from hay fever, meaning spring can be the start of watering eyes, runny noses and keeping a keen eye on the pollen forecast, which is now available and provides a look at the pollen amounts and types in the air for the next five days.

This February was the warmest on record for England and Wales and winter too has followed this wet and warm pattern. This has an influence on the pollen season ahead.

Yolanda Clewlow is the Met Office’s Relationships Manager for Health and Air Quality, and the UK Pollen Forecast Manager. She said: “We’re constantly monitoring what’s in the atmosphere and what might be coming in terms of pollen in the air. We look at a very broad picture for the pollen outlook, including the weather in the previous year when pollen is formed on some plants to make a judgement on how heavy a season might be for pollen release. Rain, wind, temperature and sunshine all affect how much pollen will be produced and how much it will be dispersed.”

The Met Office works with the University of Worcester to monitor pollen levels. There are many factors at play when predicting levels, with much depending on how much pollen has formed on the plants and the weather at the time of release. Most people are allergic to grass pollen, which is generally released around mid-May to July.

Yolanda continued: “In terms of birch pollen, we are seeing the first grains in the atmosphere in the most southerly and easterly counties but overall, there’s currently generally low levels for tree pollen due to changeable conditions. We expect the season to be of mild to average severity depending on in-season weather.

“Grass pollen could start early, with some pollen airborne from late April. Average severity is most likely, but this will largely depend on the weather in April and early May. Warm sunshine and some rain is needed for good grass growth and pollen production. Dry conditions would inhibit growth.”

There are millions of hay fever sufferers across the UK. The Met Office pollen forecasts provide vital information to help reduce the impact pollen has on their health. 

Emma Rubach is the Head of Health Advice at Asthma + Lung UK. She said: “High pollen levels can be dangerous for people with lung conditions like asthma or COPD (chronic obstructive pulmonary disease), triggering symptoms such as breathlessness, coughing or wheezing. and could lead to hospitalisation or a potentially life-threatening asthma attack.

“To protect themselves, we would advise people with hay fever to use their preventer inhaler every day as prescribed, if they have one, and always have their reliever inhaler with them, in case symptoms do arise.  

“If you’re allergic to pollen, using steroid nasal spray every day can also help, together with non-drowsy antihistamine tablets to help reduce the allergic reaction. It is also a good idea to check pollen and air pollution forecasts in their local area, so they can avoid going outdoors as much as possible on high pollen days.”    

The three types of pollen

The pollen season generally has three main pollen type phases:

  • Tree pollen – late March to mid-May
  • Grass pollen – mid-May to July (most people are allergic to grass pollen)
  • Weed pollen – end of June to September

“The first thing to do is to make sure you understand the type of pollen you’re allergic to and monitor the forecast,” said Yolanda.  

“That way, you know when the very high days of pollen are coming and you can take precautions to try and avoid the pollen. It’s not possible to avoid it altogether though so for some, it’s about taking that preventative medication and planning your day to try and avoid being out in the pollen at the peak times.”

Pets can also be affected by hay fever and in severe cases, vets can often prescribe medication to help furry friends through the summer.

Advice on dealing with hay fever is available as part of WeatherReady with the Met Office.

Pollen levels and climate change

With global temperatures increasing, the pollen season in the UK isn’t immune to the effects of a changing climate. Climate models suggest an increasing likelihood of warmer, wetter winters and hotter drier summers, which will influence the release of pollen into the atmosphere.

A study conducted by the University of Worcester looked at pollen trends in the UK over the last 26 years to better understand the relationship between these trends and meteorological factors. It examined a range of UK pollen sites, with a focus on the key pollen types: grass, birch and oak. 

Dr Beverley Adams-Groom, Senior Pollen Forecaster at the University of Worcester and lead author of the study, said: “Birch tree pollen is showing a trend for increasing severity, particularly in the Midlands region, but grass pollen is not showing an increase over time. Seasons for all pollen types are tending to start earlier and earlier but there is no evidence that season duration is changing. Where changes are occurring, these are largely related to the effects of global warming, with similar patterns seen from research in nearby countries.”

Get pollen alerts directly to your phone on the Met Office app, or visit the pollen forecast on the Met Office website.


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One in 250-year event underway high in the atmosphere.



For the first time since records began in the mid 20th century , this extended winter period (November to March) is the first in our observational records to see three Sudden Stratospheric Warming (SSW) events. Met Office research suggests the likelihood of having three SSW events in one winter period is just a one in 250-year chance, although it is more likely to happen during an El Niño winter, such as this winter.

The record of SSW events goes back to the 1950s with the introduction of radiosonde balloons which are used to take observations high in the stratosphere on a routine basis. Since then, there has been typically one SSW every two extended winters. However, there are occasional runs of years with no warming events at all e.g. in the 1990s. There are also winters with two warmings such as the winter of 2009/10.

Professor Adam Scaife, Head of Long-Range Forecasting at the Met Office, said: “Although we have not seen it before, we recently documented the chances of an unprecedented three SSW events happening in one winter. Our research work, using multiple computer simulations, showed that this could occur about once in every 250 winters.”

Professor Scaife added, “Although this is very rare, we also found that the chance of multiple SSW events is increased during El Niño and so the chance of multiple events this winter is raised.”

3rd SSW winter

Forecast pressure for mid-March showing relatively high pressure over Iceland and low pressure over mid latitudes (Units: hPa).

A SSW is a disruption of the normal westerly air flow 10 to 50 km above the earth. This often makes the jet stream meander more, which can lead to the development of a large area of high pressure over northern Europe at the Earth’s surface. This can ‘block’ the Atlantic low-pressure systems which are responsible for the relatively mild, wet and windy weather that often occurs in UK winters. This blocking pattern increases the chance of cold, dry weather in the UK and mild, wet and windy conditions for southern Europe. However, the impacts of an SSW do not always equate to cold weather, for example, we have only seen intermittent drops in temperature around the two SSW events early this winter and typically around 70% of events are associated with a cold snap.

SSW is just one of a number of global drivers that can affect weather in the UK, the current SSW is consistent with the latest long-range outlook for March which suggests a continued increased chance of blocked, high-pressure conditions to the north of the UK and a southward shift in the Atlantic jet stream.

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