3. Assessment of climate-change impacts on plant health

Climate and forecast modelling

Simulation models are a useful tool to assess the establishment, spread and damage potential of plant pests. For long-term crops such as forests, simulation models can help to quickly determine optimal management options, as well as suitable tree species and their performance under future climate conditions, to produce resilient and sustainable forest stands that are equipped for the future (Fontes et al., 2010). Models can also be a great asset for studying climate change and assessing its impacts on pests in the future.

To model climate change, scientists use the results of global climate models (GCMs; also referred to as general circulation models). These models are used to examine the effects of climate change under different greenhousegas-emission scenarios several decades into the future (NOAA, n.d.). Modellers can use GCM outputs to forecast how pests could be affected by changes in variables such as temperature or precipitation during future time periods and climate-change scenarios.

Integrated climate models

Climate models provide the data basis for most simulation models and are therefore very important. The development of the Earth’s climate is hard to project and depends on complex interactions that require enhanced climate models. With the help of these models, different pathways of climate development for different magnitudes of human emissions can be estimated. The resulting projections of future climate data can be used by other models to simulate a suite of possible outcomes of plant and pest dynamics.

Species-distribution models for plant pests potentially affected by climate change

Simulation models are used to assess the establishment and spread of pests under different conditions, including climate change. These models can be based on current species distribution (e.g. correlative statistical models) or physiological properties (e.g. mechanistic or process-oriented simulation models). They can help improve monitoring, surveillance planning, preparedness, and the determination of countermeasures, and can provide estimates of expected damage under different climate scenarios. Especially in the context of climate change, it is very important to estimate when, for instance, a (temperature) threshold is reached that will allow a pest to build long-term viable populations. In the case of species that are already present, the models can be used to estimate the pest abundance and number of generations per year.

Models can be used to predict and analyse different scenarios considering various climatic, political, and socioeconomic conditions. The main advantage of modelling is the ease with which individual parameters can be adjusted, as well as the rapid analysis of large, future time periods (Heß et al., 2020). Such a modelling approach also involves some assumptions and limitations, which need to be considered when interpreting the results (Elith and Leathwick, 2009; Kearney and Porter, 2009). For example, it is possible that effects that have not been considered influence the distribution of a pest. The calculated result can thus be under- or overestimating the actual potential distribution.

Where species-distribution models are developed for phytosanitary purposes (e.g. PRA), recommendations include the following:

• Use the best, most recent biological and climate information and data available. The data may be obtained through sources such as literature searches, experimental research, expert judgements, the knowledge systems of Indigenous Peoples, and online or internal databases.
• Edit the pest occurrence data (e.g.  remove old data points, centroids, data for misidentified pests, transient populations and interceptions, and, for plants, records from herbaria, botanical gardens, and planted populations).
• Validate the model or models by running previous time frames and known occurrence locations and compare modelling results with empirical data.
• Describe the critical assumptions, limitations, and level of uncertainty associated with models used in PRA (NAPPO, 2011).

Cultural assessments for plants and plant pests

Cultural assessments used in the modelling of impacts (damage) of pest spread and establishment on communities should recognize the rights of Indigenous Peoples under the United Nations Declaration on the Rights of Indigenous Peoples. In addition to issues of food or economic security, there are potential impacts on the identity and assets of Indigenous Peoples.

Impacts can occur through damage or loss of culturally significant plant species and includes (but is not limited to) those used in medicine, healing or well-being, ceremonies or belief systems, crafts, building and construction, or food for indigenous animals of cultural value.

Where there has been redress by governments or other agencies to address historical and socioeconomic inequalities, the impacts of climate-driven changes affecting culturally and economically important plant species that form part of any redress also need to be considered.

© FAO/Luis Tato

Climate-change pest forecasts and data sources

Climate-change pest forecasts can be useful for showing the effects of climate change on the geographical distributions of pests and characterizing future economic and environmental impacts. Potential uses for climate-change pest forecasts include strategic planning, trade discussions, modelling of longterm spread, and cost–benefit analysis (Fowler and Takeuchi, 2022).

Several climate-change datasets with future and historical baseline information are publicly available, including Köppen-Geiger (Figure 2), Multivariate Adaptive Constructed Analogs (MACA), and WorldClim (see “Additional resources” in the Bibliography). These datasets include climate parameters such as temperature and precipitation that can be used in climate-change, pest-forecasting models. Some methods for characterizing changes in pest distributions under climate change do not require complex models. For example, such changes could be estimated using suitable Köppen-Geiger zones based on where the pest occurs (see MacLeod and Korycinska (2019) for information on pest–climate matching using Köppen-Geiger zones). These approaches could be useful for NPPOs with limited resources to inform their strategic planning for future pest impacts and spread.

Horizon scanning for plants and plant pests

Horizon scanning for new and emerging planthealth threats is an important component of preventive phytosanitary activities (EFSA et al., 2021). Horizon scanning usually involves regular scans of literature, databases, pest alerts, media or any combination of these to mine new information on pests that may impact a country’s plant resources. Citizen-science platforms may also be included in scanning activities and are proving to be a useful source of information on new pest detections. Horizon scanning may be expanded to include considerations of climate-change impacts on pests (e.g. by adding new search terms in a literature search). Information of interest includes, but is not limited to: pest detections in areas that were previously climatically unsuitable; detections in neighbouring geographical areas (e.g. from the CABI Horizon Scanning Tool, the European and Mediterranean and Plant Protection Organization (EPPO) reporting service, or Pest Lens published by the NPPO of the United States of America (United States Department of Agriculture, Animal and Plant Health Inspection Service, Plant Protection and Quarantine)); new pest pathways arising as a result of climate change; new research on pest response to climatic factors; and species-distribution models that include future climate-change scenarios. By including climate change in horizon scanning, an NPPO can better identify and prepare for both new pests that are more likely to enter the country as a result of climate change and existing pests that, as a result of climate change, may pose a greater risk to plant health than they did before.

A horizon scan can be conducted to find new potential problems affecting conservation efforts, natural resources, and ecosystem services worldwide (Sutherland et al., 2011). Prediction and early detection of pests, as well as strategies of containment and eradication, are essential in preventing their further spread (Donatelli et al., 2017). Horizon scanning makes it possible to compile data on risk and impact that might help in pest management. Since horizon scanning focuses on predetermined topics of interest to the organization for which the scanning is undertaken, it can be tailored to items of interest for pest management. Additionally, the practice of horizon scanning often functions to pick up multiple pieces of information that are, in and of themselves, quite weak but collectively paint a picture that is larger than the sum of its parts. This is because such horizon scanning is managed in such a way that collected information is not assessed independently but rather aggregated and assessed alongside other topically relevant information. Such a process is carried out by the NPPO of New Zealand (Ministry for Primary Industries) and helps to inform situational awareness about multiple changes to the global biosecurity-threat environment (Marshall, forthcoming).

Figure 2: Köppen-Geiger maps for 1980–2016 (a) and 2071–2100 (b), which combine climate-change projections from 32 Coupled Model Intercomparison Project (CMIP) phase 5 models based on representative concentration pathway (RCP) 8.5.

NOTES: The Köppen-Geiger system uses seasonality values of monthly air temperature and precipitation to classify climate based on five main classes (A = tropical, B = arid, C = temperate, D = cold, E = polar) and 30 subtypes, including Af = Tropical Rainforest and Aw = Tropical Savannah (Beck et al., 2018). For more information on the Köppen-Geiger system and the criteria for each climate class, see Beck et al. (2018) and Peel, Finlayson and McMahon (2007).

Beck, H.E., Zimmermann, N.E., McVicar, T.R., Vergopolan, N., Berg, A. & Wood, E.F. 2018. Present and future Köppen-Geiger climate classification maps at 1-km resolution. Scientific Data, 5: 180214. doi.org/10.1038/sdata.2018.214

Peel, M.C., Finlayson, B.L. & McMahon, T.A. 2007. Updated world map of the Köppen-Geiger climate classification. Hydrology and Earth System Sciences, 11(5):1633–1644. doi.org/10.5194/hess-11-1633-2007

SOURCE: Beck et al., 2018. Reproduced unchanged under Creative Commons Attribution 4.0 International Licence: creativecommons.org/licenses/by/4.0/

Horizon scanning is an approach that can be used to gather information on pests, predict their potential arrival in a country and support invasive-species management. In a study conducted in Ghana, the CABI Horizon Scanning Tool was used to establish a list of potential pests that are not yet considered present in Ghana and are likely to pose a threat to agriculture and the environment. Using this list, 110 arthropods and 64 plant pathogens were evaluated using a streamlined pest risk assessment tool. Prioritization was carried out using an adapted version of the consensus method developed for ranking invasive alien species (Roy et al. 2014; Sutherland et al., 2011). At the time of assessment, 16 species had not been recorded in the African continent, of which 14 were arthropods, and two were plant diseases. Forty-six plant pathogens and 19 arthropod species were documented in Africa and found in the nearby nations of Burkina Faso, Côte d’Ivoire and Togo. In assessing the likely pathways of arrival, it was found that a large proportion of arthropod species were likely to arrive on commodities that are host plants while fewer were likely to arrive as stowaways (i.e. contaminating pests) and others were able to disperse over great distances on their own. Species with the highest scores in the prioritization exercise had a high potential for entry into Ghana because of their presence in neighbouring countries and their likelihood to establish and spread. It is possible that some of those species may already be present in Ghana but not yet detected or identified to the species level; others are probably not yet present. The main recommendations for high-scoring species included comprehensive pest risk studies as well as surveys in Ghana and neighbouring countries (Kenis et al., 2022). With these predictions on pests that pose a risk to Ghana, preventive measures, including climate-change mitigation measures, can be employed to prevent their entry, establishment and spread. An important step is to develop a contingency plan for the different stakeholders involved in pest management to follow.

Pest risk analysis

Climate suitability is an important consideration in a PRA as described in International Standard for Phytosanitary Measures (ISPM) No. 2 (Framework for pest risk analysis) and ISPM 11 (Pest risk analysis for quarantine pests).²

As climate change has the potential to affect climate suitability for pests, it should also be considered for inclusion in PRAs. Depending on the pest, the inclusion of climate-change considerations in the PRA may not be necessary (e.g. climate in the PRA already suitable) and may not be advisable (e.g. obvious that the climate will not be suitable in the near future, especially in extreme climate-change scenarios – too hot for insect species to survive). In general, the decision to include climate-change considerations in a PRA should be in line with the need for PRAs to be fit-for-purpose in aiding timely decision-making on pests and phytosanitary measures (NAPPO, 2011).

Factors to consider when deciding whether to include climate-change considerations in PRAs (see NAPPO, 2011) include the following:

• Is climate change relevant to the phytosanitary issue at hand? (See Case study 2.)
• Is the current climate in the PRA area already near the limit of climatic suitability for the pest (i.e. is it close to becoming suitable or close to becoming unsuitable), and what are the potential implications of changes in the frequency and magnitude of climate extremes on the pest?
• Could climate change increase the likelihood that the organism poses a pest risk, making such risk more certain?
• Could climate change lead to a change in the areas used to grow a particular crop or the distribution of another host that would change the pest risk associated with a particular pest?
• Is there sufficient scientific evidence to show a causal relationship between climate change and the pest risk being assessed?
• Will climate-change considerations help the NPPO decide if an organism is a pest and if phytosanitary measures are justified?

Additional guidance on when to consider climate change in PRAs for established, accidentally introduced and deliberately imported organisms is provided in section 5 of the report Integrating Climate Change into Invasive Species Risk Assessment/ Risk Management (Government of Canada, 2008): publications.gc.ca/site/eng/9.691412/ publication.html

The decision on whether or not to include climate-change considerations should be clearly summarized and documented in the PRA, and a brief explanation should be given to support that decision (NAPPO, 2011). In all PRAs that include climate data, information and references on the climate data and the time frame they cover should be clearly documented. Using the most up-to-date baseline climate data available is recommended in order to conduct the PRA based on current (or near current) climatic conditions, which will include changes in climate that have already occurred (see issues discussed in Case study 3). This recommendation highlights the need for regularly updated climate data. Depending on the phytosanitary issue at hand and the time frame for the risk assessment, using the most up-to-date data to represent current climate may be sufficient accounting for climate change (e.g. evaluating the potential of a pest to establish in the PRA area at the present time); otherwise, future climate scenarios may be included.

Current climate data can be represented by means of observational data from recent 10-, 20- or 30-year periods, but can also be represented by modelled data. Stating a default expiry date or time frame for PRAs is recommended to increase transparency and to ensure that the conclusions are not relied upon after their expected date of validity (NAPPO, 2011). Furthermore, it may be appropriate to add longer-term time horizons into the PRA process so that both current and long-term projections for pest impacts can be accounted for, while balancing the need to ensure that any phytosanitary measures taken are justifiable (NAPPO, 2011)

The type of risk assessment will affect how climate change is considered in a PRA. For a PRA initiated by the identification of a pathway (e.g. a commodity), a list of pests associated with the pathway is generated at the beginning of the assessment. At this stage, pests with marginal climate suitability (i.e. pests potentially affected by climate change) may be included in the list for further evaluation. Potential sources of information for generating the list are noted in ISPM 11 and may also include horizon-scanning activities as described above. For a PRA initiated by the identification of a pest, climate change may affect any, or even all, key elements of the PRA (entry, establishment, spread and consequences of a pest) (see Case study 1). It is important to keep in mind that climate may have different effects on the pest, its host and its vector. For details and case studies on the implications of climate change on specific elements of a PRA, see NAPPO Discussion Document DD 02: Climate Change and Pest Risk Analysis (NAPPO, 2011): nappo.org/application/ files/5415/8341/5783/DD_02_Climate_Change_ Discussion_DocumentRev-07-08-12-e.pdf

The inclusion of climate-change considerations in a PRA need not be unduly complex. Existing maps of climate-change scenarios (e.g. from the IPCC) may be combined with knowledge of the environmental requirements of a species to draw some basic conclusions. In other cases, species-distribution models that include climate-change scenarios may already be available in the published literature and can be cited in the PRA. It may also be reasonable to use studies that include models on similar, surrogate species. For example, a model of a surrogate species that shows the rate of its northward or southward movement and the average distance moved could be used to predict when a species might arrive in a PRA area if it is close but not yet directly in it. Simple, cold-threshold boundary models (e.g. based on isotherms or plant-hardiness zones) may be preferable in some cases to more complex models built on numerous assumptions. For plants, for example, climate matching using plant-hardiness maps as a broad surrogate for potential plant distribution may be a simple alternative to more complex bioclimatic models (NAPPO, 2011). The use of single-factor models such as planthardiness zones will be appropriate if the single factor is thought to be a likely limiting factor for the pest if it were introduced to the PRA area. In cases where models are developed specifically for a PRA, recommendations are provided in section 3 of this document, under “Speciesdistribution models for plant pests potentially affected by climate change”.

Climate-change considerations included in PRAs need to be sufficiently robust to meet the requirements of international agreements (e.g. World Trade Organization Agreement on the Application of Sanitary and Phytosanitary Measures; International Plant Protection Convention (IPPC)) and international case law. As such, any resulting phytosanitary measures need to be based on sufficient scientific evidence and not be arbitrary, unjustified, or a disguised barrier to trade. “Sufficient” scientific evidence should allow for an adequate risk assessment that focuses on ascertainable risk, namely, what is “likely” rather than what could be “possible”. It should also demonstrate a rational or objective relationship between a phytosanitary measure and the risk assessment (NAPPO, 2011).

In general, there is a need to intensify PRA activities as a result of climate change and its effects on pests (IPPC Secretariat, 2021a). In addition to new PRAs, existing PRAs may need revision to take into account new scientific knowledge (including biology and pest distribution) as well as change in global trade and climate-change considerations (EFSA, 2008). To deal with this increase in PRA activities, a shift from pest risk assessments of individual organisms to more generic approaches, such as pest risk assessments of groups of organisms and pathway-initiated risk analyses may be more efficient resource-wise (EFSA, 2008). Furthermore, continuous risk management may be employed to help reduce uncertainty in the PRA over the longer term and permit the integration of adaptation strategies where suitable (Council of Canadian Academies, 2022; Government of Canada, 2008). Continuous risk management is an iterative and adaptive approach to risk management that involves reexamining the PRA every few years, taking into account past iterations, updating as necessary based on new knowledge and participants, evaluating the effectiveness of mitigation measures, and readjusting as necessary (Council of Canadian Academies, 2022).

Cost–benefit analysis of pest impacts

Cost–benefit analysis of pest impacts under climate change could be a useful tool for anticipating future economic losses and strategic planning. Cost–benefit analyses have been used by NPPOs to evaluate the effectiveness of pest-control programmes to determine if they are worthwhile investments. For example, the NPPO of the United States of America conducted a cost–benefit analysis for their pine shoot beetle (Tomicus piniperda) regulatory programme (Fowler et al., 2015), which provided justification for deregulating this pest.

Similar analyses could be done using climate-change scenarios and the associated changes in pest damage over time. These types of analyses could be useful for planning and resource-allocation purposes. For example, cost–benefit analysis could be used to justify planting alternative crops or implementing safeguarding measures to reduce the likelihood of pest introduction into areas where pest impacts are expected to be high because of climate change.

Cost–benefit analysis provides a framework for gathering, assembling and presenting the data required to undertake an economic analysis of control strategies for use in a PRA. It can be used in climate-smart agricultural methods to determine how economically profitable particular methods of climate adaptation will be for smallholder farmers. A study conducted in the United Republic of Tanzania used cost–benefit analysis to investigate whether climate-smart agricultural practices would be profitable for small-scale farmers. Crop rotation and intercropping maize with early-maturing or late-maturing soybean varieties were the climate-smart agricultural techniques used. Results indicated that the techniques were financially successful (Ng’ang’a et al., 2020).

For more information on climate-smart agriculture, see Synergies and Trade-Offs in Climate-Smart Agriculture – An Approach to Systematic Assessment (FAO, 2021): doi.org/10.4060/cb5243en

Assessment of threats to culturally significant plant species

Assessment of threat levels and the cultural significance of any threat should be guided by, and preferably conducted by, Indigenous Peoples themselves. Where this is not possible, it is preferable for those making the assessments to have the endorsement of the Indigenous Peoples.

Recognition of the governance and management rights of Indigenous Peoples makes it important to include their worldviews, values and principles in the prevention of pest risk associated with climate change and in pest risk assessment. This inclusion should be upheld at all levels of decision-making and the pest risk assessment and management continuum.

Pest reporting and alert systems

The shifting of agricultural production zones has changed trade flows. However, the increase in international agricultural trade volumes will, in combination with the limited knowledge of pest behaviour under new climatic and ecosystem conditions, result in a deficiency of reliable, scientifically verifiable information upon which risk assessors and regulators can base their assessments and mitigation measures. This deficiency could be alleviated through the establishment of a reliable, international, information-exchange network dedicated to providing official services with information about the occurrence of pests and potential pathways (IPPC Secretariat, 2021a).

The official reporting of international trade pathways, pest detections and pest status is critical and should be supported by scientific research about the impacts of climate change on plant health.

The main purpose of pest reporting is to communicate immediate or potential danger. Immediate or potential danger normally arises from the occurrence, outbreak or spread of a pest that is a quarantine pest in the country in which it is detected, or a quarantine pest for neighbouring countries that are trading partners. It is also critical that information on changes to pest distribution, host range, and the adaptability of pests and host plants is shared at bilateral, regional and international levels. The IPPC reporting system (national reporting obligations (NROs)), combining official reporting by contracting parties with other available and published information from other sources, is essential for assessing and managing climate-change impacts on plant health.

Pest reports can also be made through existing RPPOs, particularly for pests potentially affected by climate change or pest-related information having impacts at a regional level.

For more information on pest reporting, see ISPM 17 (Pest reporting) — www.fao.org/3/y4224e/y4224e.pdf — and the IPPC NRO Guide (IPPC Secretariat, 2016): www.ippc.int/en/publications/80405/

For an IPPC NRO e-learning training resource, see www.ippc.int/en/ publications/91831/

For an e-learning course on surveillance and reporting obligations, see: elearning.fao.org/ course/view.php?id=824

The provision of reliable and prompt pest reports confirms the operation of effective surveillance and reporting systems within countries. Pest reporting allows countries to adjust their phytosanitary import requirements and actions to take into account any changes in pest risk. It provides useful current and historical information for the operation of phytosanitary systems.

Pest risk pathways

Increasing international trade in combination with climate change may pose major challenges and uncertainties for plant health. An increase in international trade (through regulated pathways) from countries with a warmer climate that could correspond to the future climate in importing countries means that the potential for the introduction and establishment of pests is increasing (Diez et al., 2012; Hulme, 2017). The risk of these pests expanding their geographical range and impact is likely to increase as a result of the current and predicted climatic changes (IPPC Secretariat, 2021a).

Pest dispersal occurs through both natural and regulated or unregulated pathways, strongly facilitated during recent decades by the globalization of markets for plants and plant products including food, planting material and wood. Global travel and the trade of agricultural products have moved crops and pests away from their native environments to new ones. Newly introduced crops may expand pest distribution, and the introduction of new pests into a completely new ecosystem may cause damage because pests and hosts may not have co-evolved together. This co-evolution has been especially recognized for plants and their pests (Woolhouse et al., 2002) and has created a stable balance between hosts and pests within their endemic ecosystems.

According to Anderson et al. (2004), half of all emerging plant diseases are spread by global travel and trade, while natural spread, assisted by weather events, is the second most important factor. In addition, there are also likely to be interactions between pest establishment and climatic or weather conditions. For example, global warming may facilitate the establishment of some pests that would otherwise not be able to establish (e.g. during an unusually warm winter under temperate climatic conditions) (IPPC Secretariat, 2021a).

When considering the potential impact of climate change on plant health and hence on plant distribution, it is therefore important to understand not only which conditions allow pests to thrive, but also the pathways by which they move from one place to another. An understanding of the pathways is also needed when determining what measures should be taken to mitigate and adapt to the changes in pest risk brought about by climate change.

Some ISPMs include guidance on how to conduct PRA to determine the risk of introduction (entry and establishment) and spread of pests and to select which measures to apply to prevent this occurring (ISPM 2, ISPM 11, ISPM 21 (Pest risk analysis for regulated non-quarantine pests)).

The main types of pest pathways are as follows:

For more information, see the Scientific Review of the Impact of Climate Change on Plant Pests – A Global Challenge to Prevent and Mitigate Plant Pest Risks in Agriculture, Forestry and Ecosystems (IPPC Secretariat, 2021a): doi.org/10.4060/cb4769en