More than 60 years after Rachel Carson warned of the dangers of pesticides in her book Silent Spring, the harm they are doing to wildlife may be greater than ever.

“In more or less all countries, the trend is towards increasing applied toxicity,” says at RPTU University Kaiserslautern-Landau in Germany.

The potential harm done by any pesticide depends on both the amount applied and its toxicity, which can vary greatly from species to species. To assess the overall burden of pesticides, Schulz and his colleagues have developed a measure called applied toxicity.

The researchers started by looking at the quantities of 625 pesticides used in 201 countries from 2013 to 2019. The list includes some pesticides used by organic farmers as well as conventional ones.

They then averaged data from regulators in several countries on how toxic each pesticide is to eight broad groups of organisms: aquatic plants, aquatic invertebrates, fish, terrestrial arthropods, pollinators, soil organisms, terrestrial vertebrates and terrestrial plants. This allowed the team to estimate the total applied toxicity per country or per group of organisms.

Globally, the total applied toxicity rose from 2013 to 2019 for six of the eight groups of organisms. For instance, for pollinators it rose 13 per cent, for fish 27 per cent and for terrestrial arthropods, such as insects, crustaceans and spiders, 43 per cent.

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“This does not mean that this toxicity necessarily translates into toxic actions on these organisms,” says Schulz. “But it’s at least an indicator that shows you if the pesticides we are using are more or less toxic to pollinators or to fish, or to whatever.”

Many other studies have found that the concentrations of pesticides in various environments, such as rivers, are higher than regulators assessed were possible when approving the pesticides.

“This is not included in this index, but there’s a lot of evidence,” says Schulz. There is a problem with risk assessments greatly underestimating exposure, he says.

The increases in total applied toxicity are due to two things: an increase in the quantities of pesticides being used and the replacement of older pesticides with ones that are even more toxic. In turn, this is mainly due to the evolution of resistant pests. “Resistance is, in my view, something that can only increase if you use chemical pesticides,” says Schulz.

Pesticides called pyrethroids are particularly problematic, especially for fish and aquatic invertebrates, he says, even though they are only supposed to be applied at low levels. Neonicotinoids are another problematic group, especially for pollinators.

There have been calls to ban the herbicide glyphosate, also known as the weedkiller Roundup. Although its toxicity isn’t high, because large quantities of glyphosate are used, it does contribute to the total applied toxicity, says Schulz. A ban could also backfire: the applied toxicity would increase if more toxic herbicides were used instead.

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Reducing pesticide usage more widely could have unintended consequences, too. If it reduced the productivity of farms, more farmland would be needed, leading to biodiversity loss if land is cleared.

At a UN biodiversity summit in 2022, countries agreed to reduce the by at least half by 2030. Exactly what “risk” means was never defined, says Schulz, but he thinks total applied toxicity would be one way to measure it.

The approach has its limitations, but no measure of overall pesticide use will be perfect, says at Utrecht University in the Netherlands. “Even with uncertainty, the trends it reveals are troubling,” he says. “The world is currently moving away from the UN target rather than toward it. That is bad news for ecosystems and ultimately for human health.”

“Importantly, the study also shows that a relatively small number of highly toxic pesticides drive most of the overall risk, which means there are clear, practical targets where action could have an outsized benefit,” says Vermeulen.

Transforming farming will require a broader societal shift, he says. “Consumers must be willing to accept changes in diets, reduce food waste and pay fair prices that reflect the true environmental costs of production.”

The centuries-old practice of spreading crushed limestone on farmland can improve crop yields by making soil less acidic. This custom is typically considered a source of greenhouse gas emissions, but new findings suggest “liming” may actually help remove large amounts of carbon dioxide from the atmosphere.

“Liming can be a carbon source or a carbon sink. Empirical measurements suggest this is a pretty efficient carbon sink,” says at Yale University. This could offer new motivation to spread more limestone on the world’s farms – but liming won’t have this effect everywhere.

Currently, most of the millions of tonnes of crushed limestone farmers spread on fields each year are counted as a source of emissions. That is because, as the alkaline rock dissolves in the acidic soil, much of its carbon is released as CO2. But this accounting is incomplete, says , also at Yale.

For example, soils today are very acidic due to the intensive use of fertilisers, as well as pollution from burning fossil fuels. As a result, even without crushed limestone present, other alkaline minerals found in soil will dissolve and release carbon. “Those CO2 emissions are going to occur no matter if you are putting lime into the system or not”, so added acidity, rather than liming, is to blame, says Suhrhoff.

To provide a more accurate picture of this practice’s emissions, argues Suhrhoff, researchers must compare how much CO2 is released from and taken up by the soil in scenarios with and without liming.

As an example of this approach, Suhrhoff, Planavsky and their colleagues looked at the Mississippi river basin, which collects runoff from most of the agricultural land in the US. They calculated the net carbon effect of all of the liming carried out between 1900 and 2015 in this region.

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The researchers used geochemical models, as well as data on how interventions like fertiliser and liming change soil acidity, to estimate emissions from soil. They also compared their modelling results with direct measurements of alkalinity in the Mississippi, since limestone creates alkalinity when it reacts with carbon dioxide.

Using their new approach, the researchers found liming in this region – rather than generating hundreds of millions of tonnes of emissions – actually removed about 300 to 400 million tonnes of CO2, compared to a scenario where no liming was done. Suhrhoff the work at the Goldschmidt Conference on geochemistry in Prague, Czech Republic on 10 July.

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Liming could also be paired with the growing practice of spreading crushed volcanic rocks on farms – called enhanced rock weathering – to remove even more CO2 from the atmosphere, says Planavsky.

at the Australian National University says liming can act as a carbon sink, but what worked in the Mississippi river basin won’t necessarily work everywhere. “There are risks associated with lime application that can make it a net carbon dioxide source in other systems, given strong acidification of agricultural soils,” he says.

The next steps are to identify those places where liming is most needed. “It opens up the possibility that we can incentivise something that will be good for crop yields and will potentially give us billions of tonnes of carbon dioxide removal,” says Planavsky. Such financial incentives could be particularly helpful for low-income farmers who can’t afford to do the optimal amount of liming for their crops.