Do We Need an Incubator for Disruptive Eco-Hydro-Climatology?

My contribution to UNESCO's Green Water Dialogue.

Yesterday, I had the privilege of participating in a very interesting panel discussion organized by the Intergovernmental Hydrological Program at UNESCO in Paris. The purpose of the Green Water Dialogue was to raise awareness of the active role that vegetation plays in the water cycle (“to elevate the importance of green water in the global water agenda”). More information about the event can be found here: https://unesco.org/en/articles/green-water-dialogue

Below are the questions that I responded to during the panel led by Dr. Koen Verbist, as well as some additional comments about the ensuing discussion moderated by Dr. Jacob Diamond.

Where Does the Biotic Pump Concept Stand Now?

Ms Makarieva, nearly 20 years ago you introduced the Biotic Pump concept in one of the most discussed papers in the hydrologic literature. Can you explain the origins of the concept and where it stands now?

The biotic pump concept was introduced to explain a simple but powerful idea: forests do not just use water—they help bring water to the land.

It can be understood by comparison with moisture recycling, another important ecohydrological concept that preceded the biotic pump. In this context, “recycling” refers to vegetation returning part of the precipitation back to the atmosphere through transpiration. [Moisture recycling was discussed by Dr. Åse Johannessen who opened the panel discussion.]

The moisture recycling concept assumes that winds remain the same whether the land is forested or not, and that water vapour behaves as a passive tracer. Under that view, vegetation can shift where rainfall occurs downwind, but it cannot change how much moist ocean air arrives in the first place, nor whether the incoming air ascends over land to generate rainfall. The biotic pump addresses this missing piece: how vegetation itself changes atmospheric flow.

Forests release large amounts of moisture into the atmosphere through transpiration. This moisture is not passive but profoundly shapes atmospheric dynamics. It alters air pressure and helps draw humid air inland from the ocean. In other words, forests act as an engine that sustains rainfall and river runoff far from the coast.

The mechanism behind this comes from fundamental physics. When water vapour condenses and falls as rain, the atmosphere loses mass. This creates a pressure difference at the surface that drives winds. Forests, through continuous transpiration and condensation, strengthen these pressure differences and help maintain the inflow of moist air.

Where does the concept stand now? First published as a preprint in late 2005, the biotic pump is about to stop being a teenager—and indeed, its most challenging years appear to be behind it. We can now even see (as illustrated in the keynote talk by Mr. Henk Ovink, the Executive Director of the Global Commission on the Economics of Water) that the mainstream narrative—Professor Carlos Nobre in particular—has begun to borrow biotic pump formulations, albeit still only formally, referring to the conventional notion of moisture recycling as forest water “pumps.”

To avoid confusion, it is important to bear in mind that the biotic pump is a concept distinct from moisture recycling, naturally encompassing the latter but not limited to it. One can say that the biotic pump describes how moisture recycling changes atmospheric dynamics.

Over the past two decades, many components of the biotic pump concept have been tested and published in major international journals, and it is now part of a growing scientific discussion on how ecosystems regulate climate.

In a recent paper in Global Change Biology, we identified an important dichotomy in ecosystem functioning: some dry regimes—also called landscape traps—exist where additional plant moisture does not trigger extra local rainfall and thus cannot enhance moisture import. This finding explains why mechanistic tree-planting schemes often fail as hydrological and climate solutions. For a watershed to avoid being trapped in perpetual dryness, protecting and restoring natural ecosystems is crucial, as these are uniquely capable of sustaining stable regional water cycles.

The key message for policy is that the hydrological cycle has a strong ecological dimension. Forests and other natural ecosystems are not just passive beneficiaries of climate—they are active regulators of it. To secure long-term water resilience, we must work with this complexity rather than oversimplify it. Protecting intact ecosystems and enabling natural, or close-to-natural, regeneration should be central components of water and climate policy.

[Another panel participant, Mr. Nestor Ambuy-A-Tam Musambi, a Minister Advisor in the DR Congo, emphasized that protecting forests is equivalent to enhancing water security.]

Barriers and How to Overcome Them

Ms Makarieva, your work has highlighted the need for better interaction between hydrologists, ecologists, and climate modelers. In your view, what actionable steps can we take to break down disciplinary barriers to achieve more realistic understanding of the water cycle as part of the larger biosphere?

The first question we must ask ourselves is why it is only now, a quarter into the 21st century, that we are beginning to realize the importance of ecosystems for the water cycle, and what we should do to avoid remaining at this same stage of initial realization for another few decades.

The living biosphere is an extraordinarily complex system. We have been disturbing it in many ways. Most scientific attention has focused on one type of disturbance: adding CO₂ to the atmosphere. Climate models were built primarily to diagnose this problem. They were optimized to answer one question: How will global temperature respond to rising CO₂?

For that purpose, models do perform consistently: all of them predict warming with higher atmospheric carbon dioxide, and none predict cooling. With ecohydrology, however, we see no such robustness. The effects of deforestation, land degradation, and replacing natural ecosystems with managed landscapes have never been central to climate model development. As a result, models struggle: some predict more moisture transport after deforestation, others predict less. Even the newest high-resolution models often diverge in unpredictable ways.

I suspect that few people in the audience, including hydrology experts, realize that current global climate models offer no guidance on how green water flows will change as the biosphere changes. A clear example again comes from the work of Professor Carlos Nobre, who showed a few decades ago, using a then up-to-date climate model, that Amazon deforestation would cause continental drying and savannization because of increased albedo. This mechanism is different from the biotic pump, although the predicted outcome is similar. More recently, some high-resolution climate models have projected increased rainfall in a deforested Amazon because more sensible heat is released from cleared land. [We discuss why this result is not plausible in our recent publication.]

While these contradictions among models are rarely discussed publicly, they reveal that essential physical and ecological constraints governing terrestrial hydrology are missing. Climate models are evaluated predominantly on their ability to reproduce past temperatures, not on whether they capture vegetation-driven changes in atmospheric circulation. Yet these circulation changes often matter more for water security than temperature does. A vivid example was the shift in atmospheric circulation behind the Amazon drought during the anomalously warm 2023, which none of the global climate models predicted.

Even fewer people than those aware of the lack of ecohydrological robustness in models realize that global climate models depend heavily on how they parameterize the rate at which friction dissipates wind energy.

This issue may seem totally unrelated to hydrology, but it is in fact central. It is crucial to overcome disciplinary barriers and see the whole rather than separate compartments. Friction continuously destroys exactly as much kinetic energy as the atmosphere generates through pressure gradients. These pressure gradients are directly influenced by precipitation and by the ways vegetation interacts with water. When friction processes are misrepresented in models, or mimicked without real understanding, we also misrepresent how winds respond to changes in vegetation. As a result, we lose the ability to predict water-cycle disruptions that follow vegetation degradation.

Another challenge is sociological. Disruptive science, including the biotic pump, often faces strong headwinds across disciplines. This well-documented pattern helps explain why the biotic pump has not been adopted instantly. Major breakthroughs are usually made by small teams, yet such teams today have the lowest chance of receiving support. Funding systems reward incremental work by large consortia, even when society urgently needs conceptual advances. Without pressure from policymakers and the public, the scientific system has little incentive to support ideas that challenge established assumptions and overcome entrenched disciplinary barriers.

For this reason, my colleagues and I have been advocating for something like an Global Incubator for Disruptive Eco-Hydro-Climatology: a dedicated hub for exploring innovative multidisciplinary ideas in environmental and climate science and for building new narratives around them. The biotic pump is a concrete example of how novel physical insights can sharpen constraints on how the atmosphere works and improve our understanding of the water cycle under biospheric change.

Such a centre would help hydrologists, ecologists, and climate modellers work with science communicators in a systematic way, reaching beyond disciplinary silos and creating space to think more freely, without fearing departures from long-established views. Communicators, in turn, could broaden their perspectives with new scientific horizons. This kind of joint innovation, both social and scientific, is essential for grounding water and climate policy in the real complexity of the Earth system. We need to embrace nature’s complexity and steer away from oversimplifications and one-dimensional solutions.

Discussion and Outlook

The panel discussion was followed by a Q&A session with the audience. A longer comment came from a Brazilian lawyer. He expressed concern that the new green-water agenda is being advanced using concepts he views as controversial, such as defining water as a “global common good” and recognizing the role of Indigenous lands in the water cycle—as discussed during the panel by Dr. Anna Tengberg of the International Centre for Water Cooperation in Sweden. He argued that, instead of relying on such concepts, it would be better to use existing, well-defined legal frameworks, such as national agreements on transboundary water flows. In response, it was noted that the major role of Indigenous forests in generating continental rainfall is not controversial but well established. Dr. Anna Tengberg also referred to an upcoming publication by her team.

I added that I fully agree that scientists and policymakers should work together. Regarding transboundary water flows, I clarified that these agreements apply only to the water already present in lakes, rivers, and other reservoirs. The green-water question concerns how much water will reach those reservoirs in the first place if the Amazon is deforested. It may well be that, under large-scale deforestation, there will be little or nothing left for national agreements on transboundary flows to regulate.

The UN delegate from Venezuela shared an interesting account of how her country has promoted large-scale agroforestry and achieved full independence in seeds, meaning that all seeds needed for agroforestry are now produced domestically with no imports required. She noted that Venezuela is ready to share its experience in restoring soil moisture capacity through nature-based agroforestry practices.

Finally, there was a commentary from the UN delegate from Spain, who expressed concern that green water might overshadow blue water in perceived importance. She argued that this would be problematic because societies ultimately depend on blue, or liquid, water. Spain is heavily affected by climate change, she noted, and therefore stronger emphasis should be placed on blue-water governance and on how we use the water we already have, rather than exploring what she alluded to as the less tangible realm of green water. If I understood her correctly (the intervention was translated in real time), the message was to focus on managing visible water resources rather than engaging with the “invisible” moisture flow. It was a strong and emphatic commentary.

The panel responded that green water is not a competitor to blue water but simply the other side of the same process. The real question is how much blue water Spain receives, and why. For instance, if large parts of the country become hot and dry after agricultural monocultures are harvested, then even moist air arriving from the ocean may fail to condense, leaving no blue water—much as we see today in Iran. Conversely, if Spain were to adopt agroforestry more widely and shift its landscapes toward evergreen, moisture-retaining vegetation, the amount of available blue water could increase. Dr. Åse Johannessen mentioned the work of the prominent Spanish scientist Professor Millán Millán, who investigated exactly these mechanisms in the Mediterranean region. It is time to build on this legacy.

A six-year-old agroforestry site near Entroncamento, Portugal. Chili pepper grows alongside fruit trees, legumes, and local shrubs and trees such as strawberry tree and cork oak, together with aromatic herbs. The plants flourish together with the local community (courtesy of Felipe Pasini and Dayana Andrade)

Overall, there was a strong and lively interest in the topic. One UN delegate told me informally, “Now I know what green water is. It’s not water affected by algal bloom; it’s something much more complex.” Comments like this show how much work still lies ahead in helping both ourselves and the public understand the deep interconnections between water and life. We may be entering turbulent times in which almost anything can happen, including very good things. Let us embrace the complexity of what lies ahead and steer toward respecting and preserving all Life.