The number of sheep and cattle in the UK should be reduced by between a fifth and a half to help combat climate change, a report says.

The shift is needed, the government’s advisory Committee on Climate Change (CCC) maintains, because beef and lamb produce most farm greenhouse gases.

The report foresees an increase in the number of pigs and chickens because these produce less methane.

The farm union NFU said it did not agree with reducing livestock numbers.

But environmentalists say the recommendations are too timid.

The CCC says a 20-50% reduction in beef and lamb pasture could release 3-7m hectares of grassland from the current 12m hectares in the UK.

The un-needed grassland could instead grow forests and biofuels that would help to soak up CO2.

The committee’s advice on producing less red meat is less radical than NHS Eatwell guidelines on healthy eating, which proposes a reduction in consumption of 89% for beef and 63% for lamb, and a 20% decline in dairy products.

BBC News understands that the committee have deliberately taken a more conservative position in order to minimise confrontation with the farmers’ union, the NFU.

Turning farmland into forests

The chief executive of the Committee on Climate Change (CCC), Chris Stark, told BBC News: “Climate change is going to change the way the UK looks – and we also have to alter the way we use land so we don’t make climate change worse.

“Brexit offers the government the opportunity to introduce fundamentally new policies that will reward farmers for producing less greenhouse gases and for capturing carbon emissions.”

Carbon is stored in plants and in the soil, so the CCC recommends that farm subsidies should raise the proportion of UK land under forestry from 14% to 19%, and restore peat bogs.

NFU President Minette Batters said: "The NFU has been clear with its position on British farming's role in tackling climate change - reducing livestock numbers in the UK is not a part of that policy.

"We are disappointed to see the Committee on Climate Change include that recommendation in its report. The report simply does not recognise the environmental benefits grass-fed beef and sheep production brings to the UK.

"It would be a fundamental mistake to design a farming system solely around an approach that mitigates greenhouse gases without any regard to the wider impact of such a policy for our environment and our food supply. It risks producing a one-eyed policy."

Earlier, the NFU had said that it welcomed the report's call for diversification of land use.

What do environmentalists say?

The environmental campaigner George Monbiot told BBC News: “This is a timid and inadequate report. Roughly four million hectares of uplands is used for sheep, yet sheep account for just 1.2% of our diet.

“Allowing trees to return to a significant portion on this land has a far greater potential for carbon reduction than the puny measures proposed in this report.”

Friends of the Earth's Guy Shrubsole said: "We need to reforest far more of Britain than the government's current puny tree-planting targets - going beyond what this report calls for and doubling forest cover to lock-up carbon and help prevent floods."

There may be controversy, too, over the committee’s recommendation for the UK to grow more trees and plants to burn for energy – known as biomass.

A separate CCC report says biomass can play an important role in cutting emissions in the UK – but only if it’s produced in a way that doesn’t harm the environment. It could only be used after 2030 if carbon capture equipment were fitted.

How climate change is already changing the landscape

The CCC says climate change is already changing the landscape. it says as UK temperatures have risen by 0.8°C over the last 40 years, farmers have made the impacts worse.

Loss of soil fertility, plant and animal species are now apparent, it says, mainly driven by intensive food production.

Projections suggest more warming, sea level rise, greater risks from flooding and drought.

"Despite some opportunities,” the report says,"The negative impacts on our soils, water, vegetation and wildlife are likely to be significant.”

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