To read this story in Spanish, scroll to the bottom.

Farmers in Mexico desperate for rain are asking their government to ‘bomb’ the clouds. The country is experiencing its second-worst drought in a decade, and farmers are afraid for their crops and livestock. So they’ve asked the Mexican government to use cloud-seeding technology to help them.

In March, the National Commission for Arid Zones (Conaza), a branch of the country’s agriculture ministry, announced that it would launch a rain-stimulation programme in the northeastern and northwestern states of Tamaulipas and Baja California, respectively.

The world faces a water crisis — 4 powerful charts show how

However, scientists warn that there is scant evidence that cloud seeding works, despite the Mexican government saying that it has had successes. The idea behind cloud-seeding is to disperse particles — usually crystalline silver iodide — into clouds. Because the particles have a crystal structure similar to ice, they attract water droplets to nucleate around them; eventually the droplets become heavy enough to fall as rain or snow.

Mostly, there is “theoretical evidence” that cloud seeding can increase precipitation, says Fernando García, a cloud physicist at the National Autonomous University of Mexico (UNAM) in Mexico City. Some rigorous experiments have resulted in a modest increase in precipitation. But there is no evidence that it will work every time, García says. “I can modify [a cloud]. What I don’t know is whether I’m going to increase rainfall or even suppress it, because that can happen too.”

Mexico’s Ministry of Agriculture and Rural Development (Sader) and Conaza did not respond to Nature ’s requests for comment.

Bombing the clouds

The Mexican government is working with the company Startup Renaissance to implement its cloud-seeding campaign. Alejandro Trueba, an agricultural engineer and the company’s founder and director, approached Sader in 2019, offering to tackle the country’s drought with RainMate, a silver iodide-based technology he developed to increase rainfall. A year later, Sader decided to launch its cloud-seeding efforts, and Trueba established his company.

Can artificially altered clouds save the Great Barrier Reef?

Mexico began conducting weather-modification experiments in the 1940s, García says, but research has been sparse over the decades. Since Trueba got involved, Conaza has run at least five cloud-seeding programmes, each of which has sent aircraft up into the clouds multiple times. On the basis of those runs, the agency has reported that the technology is 98% effective at mitigating the effects of drought, that it has extinguished at least 25 wildfires and that it has filled dams and aquifers.

Through a request under Mexico’s access-to-information law, Nature asked Conaza for documents showing how it evaluates the programme’s success. Conaza released 150 pages about the programme, with information from 2020 to 2022, in which it states that it is not a research or technology-regulation institution, and that it chose Startup Renaissance because the company promised efficiencies greater than 90% with its RainMate technology. The documents also show that Conaza assesses success by comparing measurements from rain gauges after cloud seeding with the amount of precipitation that had been forecast for the region beforehand.

The problem with this approach is that weather forecasts aren’t perfectly accurate. The clouds might have produced rain anyway, without the cloud seeding, García says.

Trueba confirmed to Nature that the company is “measuring [on the basis of] impacts, and not on a scientific meteorological basis” to evaluate success.

Sarah Tessendorf, a cloud physicist at the US National Center for Atmospheric Research in Boulder, Colorado, says there is some evidence that cloud seeding works. She’s the lead scientist for the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE), a project that, in 2020, reported ice-crystal growth and snowfall during three cloud-seeding runs ¹ . Tessendorf and her colleagues estimate that, over an area of approximately 2,000 square kilometres, those experiments generated an amount of snowfall equivalent to the water needed to fill 300 Olympic-sized swimming pools.

Major cloud-seeding test gives mixed results

She doesn’t suggest using cloud seeding to end a drought, however. “You have to have clouds and storms to seed to begin with,” she says.

There are ways to measure whether cloud seeding has worked, Tessendorf says. One is by using control sets of clouds — seeding some clouds but not others, where conditions are similar — and running a statistical experiment “for years and years — ideally decades”, she says.

The other way is with simulations. Computer models can now predict how clouds behave with and without seeding. Researchers compare the outputs of those models with measurements of how much water clouds hold before and after being seeded with silver iodide, she adds.

Cost-benefit analysis

In 2022, a scientific advisory committee for Mexico’s National Civil Protection System (Sinaproc) advised Conaza against implementing cloud-seeding projects without completing tasks such as a cost–benefit analysis, a verification of the technology’s success in increasing rainfall and studies to evaluate the environmental impacts of silver iodide. “Otherwise, large local, state and federal economic losses will be incurred by spending resources on weather modification projects, whose hypotheses of increased rainfall haven’t been confirmed,” it said.

After receiving the committee’s recommendation, Conaza sent a response, a copy of which Nature has received. Conaza said that, in 2021, droughts prompted farmers and livestock producers to ask the government to help cover the financial losses they expected, but that the agriculture ministry didn’t have the resources to do it. (That year, the government ended Mexico’s Agricultural Insurance Programme, which had a 2020 budget of 605 million pesos — about US$33 million — and had the goal of protecting farmers against weather-related losses). Conaza said it saw cloud seeding as a solution. It asked the committee to withdraw its recommendation, citing the programme’s successes, including filling dams with approximately 30 million cubic metres of water for human consumption.

The cloud-seeding programme cost approximately 15 million pesos in 2021, according to estimates by Conaza.

The agency did not respond to Nature ’s queries about it asking the committee to withdraw its recommendation.

Water management

Even if cloud seeding is proved to be successful, having more rain won’t solve the country’s water problems, says Abelardo Rodriguez, a water-economics consultant based in Querétaro, Mexico. Water management is also an issue. “Mexico’s population has increased dramatically, but water resources have not increased at all,” he says.

Solving Mexico’s water crisis requires a set of coordinated actions — such as improving regulations and creating a culture of conservation among citizens and industry — that are complicated by politics, he adds.

Trueba agrees. He told Nature that he would like to see the government invest in a cross-sector strategy to address the crisis.

In particular, the country could invest more in desalinating water, and in the use of water-saving irrigation systems, says Guillermo Murray, an environmental scientist at UNAM. But getting the government and its citizens to make changes will be challenging. “Behind every major technological problem, there is a social problem,” García says.

México está sembrando nubes para hacer lluvia — los científicos no están seguros de que funcione

Investigadores cuestionan la inversión gubernamental, dadas las incertidumbres sobre la tecnología.

Los agricultores en México, desesperados por tener más lluvia, están solicitando al gobierno que ‘bombardee’ las nubes. El país está experimentando su segunda sequía más severa en una década, y los agricultores temen por sus cosechas y ganado. Así que le han pedido al gobierno mexicano que use la tecnología de siembra de nubes para ayudarlos.

En marzo, la Comisión Nacional de Zonas Áridas (Conaza), una rama de la Secretaría de Agricultura del país, anunció que comenzaría a implementar un programa de estimulación de lluvias en los estados de Tamaulipas y Baja California, al noreste y noroeste del país, respectivamente.

Sin embargo, científicos advierten que hay poca evidencia de que la siembra de nubes funcione, a pesar de que el gobierno mexicano diga que ha tenido éxitos. La idea detrás la siembra de nubes es dispersar partículas — usualmente yoduro de plata cristalino — dentro de las nubes. Dado que las partículas tienen estructuras parecidas al hielo, atraen gotas de agua que se concentran en un núcleo alrededor de ellas; eventualmente, las gotas se vuelven lo suficientemente pesadas para caer en forma de lluvia o nieve.

Principalmente hay “prueba teórica” de que la siembra de nubes puede aumentar la precipitación, dice Fernando García, un físico de nubes de la Universidad Nacional Autónoma de México (UMAM), en la Ciudad de México. Algunos experimentos rigurosos han resultado en modestos incrementos en la precipitación, pero no hay evidencia de que vaya a funcionar todas las veces, dice García. “Puedo modificar [una nube]. Lo que yo no sé es si voy a aumentar la lluvia o inclusive suprimirla, porque también puede pasar esto”.

La Secretaría de Agricultura y Desarrollo Rural (Sader) de México y Conaza no respondieron a las solicitudes de comentarios de Nature .

Bombardeando las nubes

El gobierno mexicano está trabajando con la compañía Startup Renaissance para implementar su campaña de siembra de nubes. Alejandro Trueba, un ingeniero agrónomo y el fundador y director de la compañía, se acercó a Sader en 2019, ofreciendo combatir la sequía del país con RainMate, una tecnología a base de yoduro de plata desarrollada por él para incrementar la precipitación de lluvia. Un año después, Sader decidió iniciar sus esfuerzos de siembra de nubes, y Trueba estableció su compañía.

México comenzó a conducir experimentos de modificación del estado del tiempo en los años cuarenta, dice García, pero la investigación ha sido escasa a lo largo de las décadas. Desde que Trueba se involucró, Conaza ha realizado al menos cinco programas de siembra de nubes, cada uno de los cuales ha enviado múltiples veces un avión para que disperse partículas dentro de las nubes. Con base en esos programas, la institución ha reportado que la tecnología es 98 % efectiva en mitigar los impactos de la sequía; además ha extinguido al menos 25 incendios forestales y ha llenado presas y acuíferos.

A través de una solicitud de transparencia en México, Nature le solicitó a Conaza documentos que mostraran cómo la institución evalúa los éxitos del programa. Conaza entregó 150 páginas sobre el programa, con información de 2020 hasta 2022, en la que declara que no es una institución de investigación o regulación tecnológica, y que eligió Startup Renaissance porque la compañía prometía eficiencias mayores de 90 % con su tecnología RainMate. Los documentos también muestran que Conaza evalúa el éxito al comparar las mediciones captadas por pluviómetros después de sembrar las nubes con la cantidad de precipitación pronosticada en la región con antelación.

El problema con este enfoque es que los pronósticos de tiempo no son totalmente acertados. Es posible que las nubes hubiesen producido lluvia de cualquier forma sin sembrarlas, dice García.

Trueba confirmó a Nature que la compañía está “midiendo [sobre los] impactos, y no sobre una base meteorológica científica”.

Sarah Tessendorf, una física de nubes en el Centro Nacional de Investigación Atmosférica de Estados Unidos en Boulder, Colorado, dice que existe cierta evidencia de que la siembra de nubes funciona. Ella es la investigadora principal del proyecto Nubes de Invierno Orográficas Sembradas y Naturales: El Experimento de Idaho (SNOWIE), que en 2020 reportó crecimiento de cristales de hielo y caída de nieve durante tres eventos de siembra de nubes ¹ . Tessendorf y sus colegas calculan que, sobre un área de aproximadamente 2.000 kilómetros cuadrados, esos experimentos generaron una caída de nieve equivalente al agua necesaria para llenar 300 piscinas olímpicas.

Sin embargo, ella no sugiere usar la siembra de nubes para ponerle fin a una sequía. “Para empezar, necesitas tener nubes y tormentas que se puedan sembrar”, dice.

Hay formas de medir si la siembra de nubes ha funcionado, dice Tessendorf. Una es usando un grupo de control de nubes — sembrar algunas nubes, pero no otras, en donde las condiciones son similares — y correr un experimento estadístico “por años y años — idealmente décadas”, dice.

La otra forma es con simulaciones. Los modelos informáticos actuales pueden predecir cómo se comportan las nubes con y sin siembra. Los investigadores comparan los resultados de esos modelos con mediciones de cuánta agua tienen las nubes antes y después de haber sido sembradas con yoduro de plata, añade.

Análisis costo-beneficio

En 2022, un comité científico asesor del Sistema Nacional de Protección Civil (Sinaproc) le recomendó a Conaza no implementar proyectos de siembra de nubes sin antes completar una serie de tareas como un análisis de costo-beneficio, una verificación de los éxitos de la tecnología en incrementar la precipitación de lluvia y estudios para evaluar los impactos medioambientales del yoduro de plata. “De otra forma, se ocasionan grandes pérdidas económicas locales, estatales y federales al destinar recursos en proyectos de modificación del tiempo cuyas hipótesis de incremento de precipitación no han podido ser validadas”, decía la recomendación.

Después de recibir la recomendación del comité, Conaza envió una respuesta (una copia de esta fue compartida con Nature ). Conaza dijo que, en 2021, las sequías obligaron a los productores agrícolas y ganaderos a solicitar apoyos al gobierno para cubrir las pérdidas financieras que esperaban tener, pero que la secretaría no contaba con los recursos para hacerlo. (Ese año, el gobierno eliminó el Programa de Aseguramiento Agropecuario, que en 2020 contaba con un presupuesto de 605 millones de pesos mexicanos — aproximadamente 33 millones de dólares estadounidenses— y cuyo objetivo era proteger a los trabajadores del campo contra pérdidas relacionadas con el estado del tiempo). Conaza dijo que vio a la siembra de nubes como una solución. Le pidió al comité desestimar su recomendación, citando los éxitos del programa, incluidos el llenado de presas con aproximadamente 30 millones de metros cúbicos de agua para consumo humano.

El programa de siembra de nubes costó aproximadamente 15 millones de pesos mexicanos en 2021, de acuerdo con estimaciones de la Conaza.

La agencia no respondió a las preguntas de Nature sobre su solicitud al comité de retractarse en su recomendación.

Manejo de agua

Incluso si se probara que la siembra de nubes es exitosa, tener más lluvia no resolverá los problemas de agua del país, dice Abelardo Rodríguez, un consultor de economía del agua basado en Querétaro, México. El manejo de agua también es un problema. “La población de México ha aumentado dramáticamente, pero los recursos de agua no”, dice.

Resolver la crisis hídrica de México requiere un conjunto de acciones coordinadas — como mejorar las regulaciones y crear una cultura de conservación entre los ciudadanos y la industria — que son complicadas debido a la política, añade.

Trueba está de acuerdo. Le dijo a Nature que le gustaría ver al gobierno invertir en una estrategia multisectorial para resolver la crisis.

En particular, el país podría invertir más en desalinizar agua, y en el uso de sistemas de irrigación que ahorren agua, dice Guillermo Murray, un científico medioambiental de la UNAM. Pero hacer que el gobierno y sus ciudadanos implementen cambios será un desafío. “Detrás de todo gran problema tecnológico hay un problema social”, dice García.

article_text: To read this story in Spanish, scroll to the bottom. Farmers in Mexico desperate for rain are asking their government to ‘bomb’ the clouds. The country is experiencing its second-worst drought in a decade, and farmers are afraid for their crops and livestock. So they’ve asked the Mexican government to use cloud-seeding technology to help them. In March, the National Commission for Arid Zones (Conaza), a branch of the country’s agriculture ministry, announced that it would launch a rain-stimulation programme in the northeastern and northwestern states of Tamaulipas and Baja California, respectively.

The world faces a water crisis — 4 powerful charts show how

However, scientists warn that there is scant evidence that cloud seeding works, despite the Mexican government saying that it has had successes. The idea behind cloud-seeding is to disperse particles — usually crystalline silver iodide — into clouds. Because the particles have a crystal structure similar to ice, they attract water droplets to nucleate around them; eventually the droplets become heavy enough to fall as rain or snow. Mostly, there is “theoretical evidence” that cloud seeding can increase precipitation, says Fernando García, a cloud physicist at the National Autonomous University of Mexico (UNAM) in Mexico City. Some rigorous experiments have resulted in a modest increase in precipitation. But there is no evidence that it will work every time, García says. “I can modify [a cloud]. What I don’t know is whether I’m going to increase rainfall or even suppress it, because that can happen too.” Mexico’s Ministry of Agriculture and Rural Development (Sader) and Conaza did not respond to Nature’s requests for comment. The Mexican government is working with the company Startup Renaissance to implement its cloud-seeding campaign. Alejandro Trueba, an agricultural engineer and the company’s founder and director, approached Sader in 2019, offering to tackle the country’s drought with RainMate, a silver iodide-based technology he developed to increase rainfall. A year later, Sader decided to launch its cloud-seeding efforts, and Trueba established his company.

Can artificially altered clouds save the Great Barrier Reef?

Mexico began conducting weather-modification experiments in the 1940s, García says, but research has been sparse over the decades. Since Trueba got involved, Conaza has run at least five cloud-seeding programmes, each of which has sent aircraft up into the clouds multiple times. On the basis of those runs, the agency has reported that the technology is 98% effective at mitigating the effects of drought, that it has extinguished at least 25 wildfires and that it has filled dams and aquifers. Through a request under Mexico’s access-to-information law, Nature asked Conaza for documents showing how it evaluates the programme’s success. Conaza released 150 pages about the programme, with information from 2020 to 2022, in which it states that it is not a research or technology-regulation institution, and that it chose Startup Renaissance because the company promised efficiencies greater than 90% with its RainMate technology. The documents also show that Conaza assesses success by comparing measurements from rain gauges after cloud seeding with the amount of precipitation that had been forecast for the region beforehand. The problem with this approach is that weather forecasts aren’t perfectly accurate. The clouds might have produced rain anyway, without the cloud seeding, García says. Trueba confirmed to Nature that the company is “measuring [on the basis of] impacts, and not on a scientific meteorological basis” to evaluate success. Sarah Tessendorf, a cloud physicist at the US National Center for Atmospheric Research in Boulder, Colorado, says there is some evidence that cloud seeding works. She’s the lead scientist for the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE), a project that, in 2020, reported ice-crystal growth and snowfall during three cloud-seeding runs1. Tessendorf and her colleagues estimate that, over an area of approximately 2,000 square kilometres, those experiments generated an amount of snowfall equivalent to the water needed to fill 300 Olympic-sized swimming pools.

Major cloud-seeding test gives mixed results

She doesn’t suggest using cloud seeding to end a drought, however. “You have to have clouds and storms to seed to begin with,” she says. There are ways to measure whether cloud seeding has worked, Tessendorf says. One is by using control sets of clouds — seeding some clouds but not others, where conditions are similar — and running a statistical experiment “for years and years — ideally decades”, she says. The other way is with simulations. Computer models can now predict how clouds behave with and without seeding. Researchers compare the outputs of those models with measurements of how much water clouds hold before and after being seeded with silver iodide, she adds. In 2022, a scientific advisory committee for Mexico’s National Civil Protection System (Sinaproc) advised Conaza against implementing cloud-seeding projects without completing tasks such as a cost–benefit analysis, a verification of the technology’s success in increasing rainfall and studies to evaluate the environmental impacts of silver iodide. “Otherwise, large local, state and federal economic losses will be incurred by spending resources on weather modification projects, whose hypotheses of increased rainfall haven’t been confirmed,” it said. After receiving the committee’s recommendation, Conaza sent a response, a copy of which Nature has received. Conaza said that, in 2021, droughts prompted farmers and livestock producers to ask the government to help cover the financial losses they expected, but that the agriculture ministry didn’t have the resources to do it. (That year, the government ended Mexico’s Agricultural Insurance Programme, which had a 2020 budget of 605 million pesos — about US$33 million — and had the goal of protecting farmers against weather-related losses). Conaza said it saw cloud seeding as a solution. It asked the committee to withdraw its recommendation, citing the programme’s successes, including filling dams with approximately 30 million cubic metres of water for human consumption. The cloud-seeding programme cost approximately 15 million pesos in 2021, according to estimates by Conaza. The agency did not respond to Nature’s queries about it asking the committee to withdraw its recommendation. Even if cloud seeding is proved to be successful, having more rain won’t solve the country’s water problems, says Abelardo Rodriguez, a water-economics consultant based in Querétaro, Mexico. Water management is also an issue. “Mexico’s population has increased dramatically, but water resources have not increased at all,” he says. Solving Mexico’s water crisis requires a set of coordinated actions — such as improving regulations and creating a culture of conservation among citizens and industry — that are complicated by politics, he adds. Trueba agrees. He told Nature that he would like to see the government invest in a cross-sector strategy to address the crisis. In particular, the country could invest more in desalinating water, and in the use of water-saving irrigation systems, says Guillermo Murray, an environmental scientist at UNAM. But getting the government and its citizens to make changes will be challenging. “Behind every major technological problem, there is a social problem,” García says. vocabulary:

{'Cloud seeding': '云种播：一种技术，通过向云中撒放微粒（通常是结晶银碘化物）来增加降水量', 'Nucleate': '核状：形成核状结构', 'Crystalline': '晶体：晶体结构的', 'Silver iodide': '银碘化物：一种化合物，用于云种播', 'Precipitation': '降水：从大气中落下的水滴', 'Orographic': '山地：山地的', 'SNOWIE': '种植和自然山地冬季云：爱达荷实验：一个项目，于2020年报告了三次云种播运行期间的冰晶生长和降雪', 'Simulations': '模拟：用计算机模拟', 'Statistical': '统计：基于统计学的', 'Control sets': '控制集：一组控制变量', 'Cost–benefit analysis': '成本效益分析：一种经济学分析，用于评估投资的成本和收益', 'Verification': '验证：确认或证实', 'Environmental impacts': '环境影响：对环境的影响', 'Desalinating': '脱盐：去除盐分的过程', 'Irrigation systems': '灌溉系统：用于提供水分的系统', 'Regulations': '规定：法规或规章', 'Conservation': '保护：保护自然资源的行为', 'Coordinated actions': '协调行动：协调一致的行动', 'Politics': '政治：政治活动', 'Cross-sector': '跨部门：跨越不同部门的', 'Desalinating': '脱盐：去除盐分的过程', 'Irrigation systems': '灌溉系统：用于提供水分的系统', 'Regulations': '规定：法规或规章', 'Conservation': '保护：保护自然资源的行为', 'Coordinated actions': '协调行动：协调一致的行动', 'Politics': '政治：政治活动', 'Cross-sector': '跨部门：跨越不同部门的'} readguide:

{'reading_guide': '这篇文章讲述了墨西哥农民为了缓解旱情而要求政府使用云种植技术，但科学家警告说，没有证据表明云种植技术有效。文章还提到，解决墨西哥水危机需要一系列协调行动，但由于政治原因，这些行动很难实施。本文指出，解决墨西哥水危机的方法包括改善法规，在公民和工业界建立节约文化，以及投资淡化水和节水灌溉系统。'} long_sentences:

{'sentence 1': 'Mexico began conducting weather-modification experiments in the 1940s, García says, but research has been sparse over the decades.', 'sentence 2': 'Solving Mexico’s water crisis requires a set of coordinated actions — such as improving regulations and creating a culture of conservation among citizens and industry — that are complicated by politics, he adds.'}

Sentence 1: 墨西哥在1940年代就开始进行气象改造实验，但是García说，几十年来研究一直很稀少。

Sentence 2: 解决墨西哥水危机需要一系列协调行动，比如改善法规，在公民和行业之间建立节约文化，但这些行动受到政治的干扰。