This warmer environment alters plants so they are tougher to digest, and so each mouthful spends more time in the animals' stomach, producing more methane, further warming the planet, and the cycle continues.
Plants growing in warmer conditions are tougher and have lower nutritional value to grazing livestock, inhibiting milk and meat yields and raising the amount of methane released by the animals.
That's because more methane is produced when plants are tougher to digest - an effect of a warmer environment.
Methane is a potent greenhouse gas, around 25 times better at trapping heat than carbon dioxide over a century, and 85 times stronger over 20 years.
More than 95% of the methane produced by cows comes from their breath through eructation (belching) as they chew the cud.
The findings come in a published a paper today in the journal Biogeosciences by scientists at the Royal Botanic Gardens, Kew, Scotland's Rural College (SRUC) and the Senckenberg Biodiversity and Climate Research Centre, Frankfurt.
The key finding is a near doubling of ruminant emissions of methane: "Upscaling the GHG footprint of the current livestock inventory to the 2050 projected inventory increases annual GHG emissions from enteric sources from 2.8 to 4.7 GT CO2eq."
Dr Mark Lee, a research fellow at the Royal Botanic Gardens, Kew who led the research says; "The vicious cycle we are seeing now is that ruminant livestock such as cattle produce methane which warms our planet.
"This warmer environment alters plants so they are tougher to digest, and so each mouthful spends more time in the animals' stomach, producing more methane, further warming the planet, and the cycle continues. We need to make changes to livestock diets to make them more environmentally sustainable."
Harsher climate makes tougher plants
There are several reasons why rising temperatures may make plants tougher for grazing livestock to digest. Plants have adaptations to prevent heat damage, they can flower earlier, have thicker leaves or in some cases, tougher plants can invade into new areas replacing more nutritious species - all of which makes grazing more difficult.
This is a pressing concern, because climate change is likely to make plants tougher for grazing cattle, increasing the amount of methane that the animals breathe out into the atmosphere.
The researchers mapped the regions where methane produced by cattle will increase to the greatest extent as the result of reductions in plant nutritional quality. Methane production is generally expected to increase all around the world, with hotspots identified in North America, Central and Eastern Europe, and Asia, where the effects of climate change may be the most severe.
Many of these regions are where livestock farming is growing most rapidly. For example, meat production has increased annually by around 3.4% across Asia, compared with a more modest 1% increase across Europe.
The calculations, write the scientists, "suggested a previously undescribed positive climate change feedback, where elevated temperatures reduce grass nutritive value and correspondingly may increase methane production by 0.9% with a 1C temperature rise and 4.5% with a 5C rise (model average), thus creating an additional climate forcing effect.
"Future methane production increases are expected to be largest in parts of North America, central and eastern Europe and Asia, with the geographical extent of hotspots increasing under a high emissions scenario."
Act now to limit the damage!
Global meat production has increased rapidly in recent years to meet demand, from 71 million tonnes in 1961 to 318 million tonnes in 2014, a 78% increase in 53 years (FAOSTAT, 2016). Grazing lands have expanded to support this production, particularly across Asia and South America, and now cover 35 million km2; 30% of the Earth's ice-free surface.
However, livestock are valuable. They are worth in excess of $1.4 trillion to the global economy and livestock farming sustains or employs 1.3 billion people around the world (Thornton, 2010). The upward trend in livestock production and associated GHG emissions are projected to continue in the future and global stocks of cattle, goats and sheep are expected to reach 6.3 billion by 2050 (Steinfeld et al. 2006).
If these rises are to continue then the researchers say it will be necessary to limit the growth of livestock farming in the most rapidly warming regions, and, to avoid significant losses in livestock production efficiency and increases in methane emissions. Other measures, including eating less meat and farming more sustainably, are also essential:
"A global switch in human diets and a transition to more sustainable agricultural practices, as well as a greater prevalence of organic and silvopastoral farming, may reduce our reliance on intensively farmed cattle and other ruminants.
"In countries with high or increasing meat consumption, these measures could reduce the environmental impacts of agriculture and contribute to GHG emissions cuts with an associated improvement in human health."
And the authors emphasise that we need to start implementing policy measures as soon as possible. "Now is the time to act," said Dr Lee, "because the demand for meat-rich diets is increasing around the world. Our research has shown that cultivating more nutritious plants may help us to combat the challenges of warmer temperatures.
We are undertaking work at Kew to identify the native forage plants that are associated with high meat and milk production and less methane, attempting to increase their presence on the grazing landscape. We are also developing our models to identify regions where livestock are going to be exposed to reductions in forage quality with greater precision.
It is going to be important to put plans in place to help those countries exposed to the most severe challenges from climate change to adapt to a changing world."
Oliver Tickell is contributing editor at The Ecologist.
The paper: 'Forage quality declines with rising temperatures, with implications for livestock production and methane emissions' is by Lee, M.A., Davis, A.P., Chagunda, M.G.G., Manning, P. and published in Biogeosciences 14, 1403-1417, doi:10.5194/bg-14-1403-2017.