Muhammad Hammad

The United Nations Environment Program is mandated to keep the environment under review, which means monitoring environmental changes and issues that may impact our shores. The Frontiers report is a key component of this effort, uniting scientists and specialists from around the world to explore critical emerging environmental issues and recommend policies and courses of action. The 2025 edition of this report addresses four issues that need greater attention from policymakers.

1. The frozen pandora’s box: Reactivation of microbes in a warming cryosphere

The first issue is the threat posed by reactivated microbes in a warming cryosphere. Frozen in ice sheets, glaciers, and permafrost are bacteria, fungi, and viruses. While most are dead, some are dormant, and some are active. As global temperatures hit record highs, these microorganisms will become more active in many ecosystems. Even if the melt can be slowed down by mitigating greenhouse gas emissions, we must assess and prepare for possible threats from potential pathogens. Also crucial is documenting and preserving cryosphere microorganisms, which can shed light on the history of climate and evolution, help in finding therapies for diseases, and develop innovative biotechnologies.  

Life below zero

With humanity’s release of greenhouse gas emissions over the last 250 years, our planet is warming at an unprecedented rate. In the previous 50 years, the global average surface temperature has increased at a rate faster than any previous 50-year period in the last 2,000 years.

Among terrestrial and aquatic ecosystems, the cryosphere is one of the most affected by climate change, suffering substantial damage and irreversible losses. The cryosphere refers to regions where water is frozen solid and ground is seasonally frozen: The Arctic region, the Antarctic and Greenland ice sheets, ice shelves and sea ice that extend into the ocean, mountain glaciers on every continent, permafrost, seasonally frozen ground, and other ice and snow-covered features.

Thawing cryosphere and reactivated microorganisms

Between 1994 and 2017, 28 trillion tonnes of ice vanished from the Earth’s cryosphere, and, compared to the 1990s, the overall rate of ice loss increased by 57 per cent over those 24 years. Projections to 2100 suggest that the number of the world’s glaciers will be halved, even if temperature increase could be limited to +1.5°C. Glaciers are no longer shrinking – they are disappearing. Similarly, recent estimates suggest that 24 to 69 per cent of the planet’s near-surface permafrost will thaw by 2100.

Cryosphere warming will reactivate and remobilize modern and ancient microorganisms to new terrestrial and aquatic environments. Once there, specific populations of re-emerging microorganisms might thrive, profoundly modifying the structure and function of the existing microbial communities and surrounding ecosystems. A 2022 estimate suggests that 2.9 x 1022 microbes will be discharged annually into downstream ecosystems in the Northern Hemisphere for the next 80 years due to glacier melting. On the other hand, some cryosphere microorganisms will not survive the thaw, resulting in a loss of both microbial diversity and the potentially valuable information contained within their genomes.

2. Clearing the path: Barrier removal for river restoration

The second issue is the growing need to remove barriers, such as dams, to rehabilitate river ecosystems. Assessing the success of barrier removal in restoring ecological health to rivers depends on the metrics used to evaluate ecological health and how success is defined. When barriers are removed, river connectivity is restored allowing water, sediment, nutrients, and organisms to move up and downstream. One of the major drivers of barrier removal is to restore long-distance migration pathways for fish, particularly species that swim from the ocean to spawn in rivers. Upon removal of a barrier, the upstream passage opens almost immediately, so fish and other mobile species benefit from access to tens or hundreds of kilometers of formerly unavailable upstream habitats and can rapidly recolonize and repopulate.

Removing barriers to healthy rivers  

However, in some cases, the abundance of aquatic plants and animals may initially decline before their populations increase. When the reservoir drains, conversion from a lake-like ecosystem back to a free-flowing riverine system can drive fundamental shifts in aquatic communities from species that favor lakes and reservoirs to those adapted to flowing waters. Biological response may be rapid, but the long-term ecological recovery may not settle at pre-dam conditions.

Biological recovery tends to lag behind sediment redistribution. Once connectivity is restored, the downstream flow of sand, silt, and clay resumes. Eventually, the renewed sediment delivery will support the revival of habitats for fish and invertebrates and reverse the legacy of erosion along the river, in riparian areas, and in some cases, coastal beaches.

Momentum gained for river restoration

Barrier removal is gaining momentum in Europe and North America to restore river ecosystems. The European Union aims to free 25,000 km of rivers from barriers by 2030. While barrier removal can improve river health and public safety, it often sparks controversy due to economic, cultural, and recreational concerns. Stakeholder engagement and collaboration are crucial for successful project implementation. Removal can prevent disasters, reduce maintenance costs, and adapt to climate change.

3. Demographic challenge: Growing old in a changing environment

The third issue is the risk to ageing populations from environmental degradation. It is estimated that the global share of people over 65 years old will rise from 10 per cent in 2024 to 16 per cent by 2050. Most of these people will live in cities, where they will be exposed to extreme heat and air pollution and experience more frequent disasters. Older people are already more at risk. Effective adaptation strategies will need to evolve to protect these older populations.

Demographic transitions and ageing populations

Older people are especially vulnerable to weather changes. Extreme heat can be deadly for older populations, given their reduced ability to regulate body temperature. Their vulnerability can be tracked as a pattern, where older people experience high rates of illness and death during extreme heat and cold events among various age groups. An analysis of mortality due to extreme temperatures in 326 cities across Latin America from 2002 to 2015 estimated that 7.6 per cent of deaths in older populations were associated with extreme cold and, to a lesser extent, heat. A study of heatwave-related mortality in China from 1979 to 2020 indicated that people over 75 accounted for 55 per cent of all heat-related deaths. Vulnerability to heat also increases among those living in social isolation and overcrowded urban areas.

Environmental risks for ageing populations

With climate change, we can expect more intense and more frequent heat waves. According to projections of heat stress on humans, combining air temperature with relative humidity, the exposure to dangerous heat levels will likely double for people living in the tropics and reach 3 to 10 times for some living in the mid-latitudes. A study of future exposure to extreme heat events in the Republic of Korea suggests that, by 2060, older people in its cities will be four times more exposed to extreme heat than today.

Inclusive resilience for the ageing population

A healthy environment is crucial for active and healthy ageing. Creating age-friendly cities with low pollution, green spaces, and accessible facilities promotes quality of life for all ages. Concepts like “ageing in place” and “15-Minute City” emphasize the importance of local, accessible, and sustainable neighborhoods, enabling older individuals to live independently and maintain their well-being.

4. Forgotten but not gone: Remobilization of legacy pollutants by flood events

The final issue is the danger posed by legacy pollutants released as extreme rainfall and floods wash away sediments and debris. The Pakistan floods of 2010, flooding in the Niger Delta in 2012, and Hurricane Harvey off the coast of Texas in 2017 are all examples when floodwaters stirred up sediments, releasing heavy metals and persistent organic pollutants. Evaluating sediments to understand hazards, rethinking flood protection to lean on Nature-based Solutions, and investments in natural remediation of contaminated sediments are all options to deal with this problem.

After a deluge

In recent years, many regions of the world have faced an increase in the frequency and magnitude of severe storm events with extreme rainfall and consequential floods. Bangladesh, a country frequently assailed by natural disasters, was struck by cyclones such as Mora in 2017, Amphan in 2020, and Yaas in 2021, which led to massive inundations.1-3 Southeast Asia witnessed high-magnitude floods in 2011 that affected multiple countries, with particularly severe economic impacts in Cambodia, the Laos, Malaysia, Myanmar, the Philippines, Thailand, and Viet Nam. In China, substantial rains in June 2020 triggered some of the most devastating floods experienced in over two decades. Similarly, Australia grappled with extreme flooding following intense droughts in early 2020. Some parts of the country received a year’s rainfall within a week.

East Africa was not spared either, with the 2019 and 2024 floods disastrously affecting the region’s populations and ecosystems. Unprecedented rainfall produced by Hurricane Ida in September 2021 caused one of the most extreme floods in the eastern United States of America. The heavy rainfall of 2021 in Europe transformed even smaller rivers and streams in Austria, Belgium, France, Germany, Italy, Poland, and Romania into raging torrents. These flood events caused severe injuries and fatalities, temporary or permanent displacement of people, and substantial infrastructure and economic damage.

Scientific evidence indicates that global climate change is a primary driver behind the increased amount and intensity of rainfall. As average global temperatures rise, extreme rainfall will likely amplify the severity of flooding in many regions. The risk of flooding is further magnified by poor land-use practices, burgeoning populations in urban and coastal regions, and the increasing economic value of flood-prone areas in many countries. Without adaptive measures, global flood damage by the end of this century could be up to 20 times more severe.

Flood management and adaptation

Flood management initiatives worldwide prioritize different approaches based on regional circumstances. Europe focuses on flood retention, Bangladesh on tidal river management, Mozambique on drainage system rehabilitation, China on nature-based solutions like “sponge cities,” and Japan on combining hard engineering with non-structural measures. The Netherlands’ “Room for the Rivers” approach is a collaborative initiative to reduce flood risk by increasing flood detention and drainage capacity. These diverse approaches aim to minimize flood impacts on human health, environment, and economy.