How much methane does a cow actually produce?

Cows are notorious for the amount of methane they produce. Methane is a powerful greenhouse gas (GHG), but just how much do cows actually give off and how does this compare to other methane emission sources? This post tries to give an overview of all things methane and cows..

Where does the methane come from?

The plant diet of cows and other ruminants is high in cellulose, which cannot be digested by the ruminant itself. However, ruminants have a symbiotic relationship with colonies of microorganisms, called methanogens, which live in their gut and break down the cellulose into carbohydrates.. These carbohydrates provide both the microbial community and the ruminant with an energy source. Methane is produced as a by-product of this process.

A common misconception is that the cow’s rear end emits methane, however the vast majority is released orally. Researched carried out by Grainger et al. in 2007 found that 92-98 % was emitted orally (I won’t go into detail about how they found that out!). It is also wrong to think that all bovines emit the same amount of methane but I will go into this in more detail later on in the post.

Global Bovine Emissions

Global emissions of methane were estimated to be between 76 – 92 Tg per year (1 Tg = 1 million metric tonnes). This is roughly equal to ~10-15 % of global methane emissions, which in turn is ~15 % of global GHG emissions. Methane is a more potent GHG than CO2, which means that gram for gram methane warms the atmosphere more than CO2. Methane also has a much shorter lifetime in the atmosphere compared to CO2 (~10 years compared to 100s of years) which will produce more rapid impacts on the global climate. This also means that any reductions in methane emissions will see a faster decrease in atmospheric concentrations than compared to CO2.

Dairy vs. Beef (and some other animals)

The table and figure below compares farmed animals in the UK and their contribution to methane emissions per animal per year in kilograms. A dairy cow emits over twice the amount of methane than a beef cow and is by far the highest contributor of all the animals studied. There are also more dairy cows in the UK than beef cattle (1.81 million compared to 1.66 million). All data found from the UK GHG Inventory report 1990-2012. Other research has shown that cows emit methane at regular times of the day, specifically during feeding and milking. Although these figures do not take into account farming and transport GHG emissions and the actual amount of milk each animal produces, perhaps it would be better to buy goat milk? Although the more preferable metric of Carbon per Litre would allow a more concrete conclusion on this point.

Kg CH4 # Dairy Cows
Pigs 1.5 74
Goats 5 22
Sheep 8 14
Beef 50.5 2
Dairy 110.7 1
Graphics credit to @OatJack.

Graphics credit to @OatJack.

Future Bovine Emissions

Future methane emissions are almost certainly expected to increase due to global food demand increasing from population growth. Developed nations also consume more meat, developing nations are thus expected to increase their meat consumption in future years.

Methods to reduce methane emissions from cows are summarised in the table below (taken from Reay’s book Methane and Climate Change). These have been classified into short term (available now), medium term (available in ten years) and long term (not commercially available for at least another ten years). Many of these suggestions have been disputed as they are not economically viable, especially in developing nations.

Short Term Medium Term Long Term
Reduce animal numbers Rumen Modifiers Targeted manipulation of rumen ecosystem
Increase productivity per animal Select plants that produce lower methane yield by the animals Breed animals with low methane yield
Manipulate diet
Rumen Modifiers

An alternate pathway would be to try and capture emissions from cows. A dairy cow can produce up to 400 litres of methane per day! When burned, this is enough energy to power a small fridge for a day. Some scientists have harnessed methane emitted from cows in backpacks (see photo and video below) however scaling this up to all 10 million cows (this includes all calves, young bulls etc in the UK alone) could be problematic!


Methane backpack. Image sourced from here:

Click for the video: “Backpacks measure cows’ methane” (BBC News)

If you have any questions concerning this post please ask in the comments below!


Key Findings from the IPCC Working Group 2: Impacts & Risks from Climate Change


The IPCC (Intergovernmental Panel on Climate Change) consists of three working groups who publish reports every 6 years describing the current understanding of all aspects of climate change (CC). The previous blog post (Key Figures from the IPCC’s AR5 Report) gives an introduction into the IPCC and explains the key figures published by the First Working Group last winter which concentrates on the scientific evidence of CC. This blog post summarises the highlights from the Second Working Group (WG2) which aimed to assess the risks associated with CC, particularly Impacts, Adaptation and Vulnerability.

WG2’s Summary for Policy Makers, which this post is based upon, assesses relevant scientific, technical and socioeconomic literature. Such literature is comprised of empirical observations, experimental results, process-based understanding, statistical approaches, simulation and descriptive models as well as expert judgement.

NB: For descriptions of the confidence and certainty values mentioned in this post please see this previous post on the WG1 report.

Current Observed Impacts, Vulnerability, and Exposure

The figure below summarises the observed impacts around the world in the last few decades. A hollow symbol means that the evidence suggests CC has had a minor impact, and a filled symbol represents a major CC contribution. The rectangular bars beside each symbol represent the confidence associated with each impact. A summary of the major themes found throughout the impacts stated on this map are listed below. All statements are stated to have high or medium confidence levels.

  • Changes in precipitation or melting snow / ice is altering some regions’ hydrological systems, which can affect water resources and quality.
  • Many species have shifted their geographic ranges, seasonal migration times, abundances etc. A few species have become extinct due to current CC.
  • Negative impacts of CC on crop yields currently outweigh positive impacts. Positive impacts are mainly localised to the Northern hemisphere higher latitude regions.
  • Currently, the impact of CC on worldwide human health is minimal when compared with other contributing factors. It should be noted that there has been an increase in heat related mortality and a decrease in cold related mortality.
  • Climate extremes, for example, heat waves, droughts, floods (which are expected to increase in the future) are a significant vulnerability for some ecosystems.
  • Climate related hazards aggravate other stressors that impact human life, especially for people living in poverty. For example, food price increases due to lower crop yields. Observed positive impacts are limited and often indirect, for example, diversification of agricultural practices.
Widespread impacts in a changing world. (A) Global patterns of impacts in recent decades attributed to climate change, based on studies since the AR4. Impacts are shown at a  range of geographic scales. Symbols indicate categories of attributed impacts, the relative  contribution of climate change (major or minor) to the observed impact, and confidence in  attribution.

Widespread impacts in a changing world. (A) Global patterns of impacts in recent decades attributed to climate change, based on studies since the AR4. Impacts are shown at a range of geographic scales. Symbols indicate categories of attributed impacts, the relative contribution of climate change (major or minor) to the observed impact, and confidence in attribution.

The following diagram shows regional examples of the impacts described in the previous paragraph. For a more comprehensive list of impacts please see the Summary for Policy Makers Table 1. All statements listed below have confidence levels shown in brackets after each impact.

 Constructed using data from the WG2 SPM Table 1.

The figure below shows the average percentage change in crop yield. The two blue bars to the left show all crops grouped as growing in tropical or temperate climates. The four orange bars on the right are the most common crops worldwide. It has been observed on the whole that CC has had a negative impact on yields of most crops worldwide.

(C) Summary of  estimated impacts of observed climate changes  on yields over 1960-2013 for four major crops in temperate and tropical regions, with the  number of data points analyzed given within parentheses for each category.

(C) Summary of estimated impacts of observed climate changes on yields over 1960-2013 for four major crops in temperate and tropical regions, with the number of data points analyzed given within parentheses for each category.

Risks in the Future

The second half of the summary for policy makers concentrates on the future risks and possible benefits CC will bring to the world. The magnitude and rate of CC is also taken into consideration. The IPCC Working Group 1 used four representative concentration pathway (RCP) scenarios to predict the average global temperature leading up to 2100. For definitions of the RCP scenarios please read my last post. The following risks predicted by the WG2 all occur in at least one of the RCP scenarios and all possess high confidence levels.

  • Risk of death, injury, ill-health, or disrupted livelihoods in low-lying coastal zones and small island developing states and other small islands, due to storm surges, coastal flooding and rising sea level.
  • Risk of severe ill-health and disrupted livelihoods for large urban populations due to inland flooding in some regions.
  • Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services such as electricity, water supply, and health and emergency services.
  • Risk of increases in mortality and disease during periods of extreme heat, particularly of vulnerable urban populations and those working outdoors.
  • Risk of food insecurity and the breakdown of food systems linked to warming, drought, flooding, and precipitation variability and extremes, particularly for poorer populations in urban and rural settings.
  • Risk of loss of rural livelihoods and income due to insufficient access to drinking and irrigation water and reduced agricultural productivity, particularly for farmers and pastoralists with minimal capital in semi-arid regions.
  • Risk of loss of marine and coastal ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for coastal livelihoods, especially for fishing communities in the tropics and Arctic.
  • Risk of loss of terrestrial and inland water ecosystems, biodiversity, and the ecosystem goods, functions, and services they provide for livelihoods.
  • Many key risks constitute particular challenges for the least developed countries and vulnerable communities, given their limited ability to cope.

Future Risks by Sector

  • Freshwater resources: Risks increase significantly as concentrations increase (robust evidence, high agreement). Renewable surface- and groundwater resources are expected to decrease over the 21st century especially in subtropical dry regions which would intensify competition for water supply (limited evidence, medium agreement).
  • Terrestrial & freshwater ecosystems: Both ecosystems are expected to have a large fraction facing extinction risks during and beyond the 21st century. This is because CC can have an impact on other factors such as pollution, invasive species etc. (high confidence). There is also the risk of abrupt and irreversible regional-scale change in the higher RCP scenarios (medium confidence).
  • Coastal & low-lying areas: Rising sea levels are predicted throughout the 21st century. Coastal and low-lying areas will therefore increasingly experience adverse impacts such as flooding and coastal erosion (very high confidence).
  • Marine systems: Global marine-species redistribution and marine-biodiversity reduction will challenge the sustained provision of fisheries productivity (high confidence). For medium to high RCPs, ocean acidification will cause substantial risks particularly to coral reefs and polar regions (medium – high confidence).
  • Food security & food productions systems:  Major crops will see a negative impact on production without adaptation, however individual locations may benefit (medium confidence). All aspects of food stability are affected by CC, including food access and price stability (high confidence).
  • Urban areas: Urban areas are affected by many CC risks (medium confidence). Risks are amplified for those lacking appropriate infrastructure, those in poor quality housing and in exposed areas (medium confidence).
  • Rural areas: Rural areas are exposed to both near- and long-term risks from CC. These impacts include water availability, supply and food shortages (high confidence).
  • Key economic sectors & services: For economic sectors, other stressors, including population, technology, relative prices etc. will be larger than the impacts of CC (medium confidence). Global economic impacts from CC are very difficult to estimate. The most recent, but still incomplete, estimates predict an increase of ~2 oC would have a negative economic effect of between 0.2 and 2 % of income (medium evidence, medium agreement).
  • Human health: Until mid-century CC will impact human health by exacerbating current health issues (very high confidence). It is expected to increase throughout the 21st century in many regions, especially developing countries with low income (high confidence).
  • Human security: The displacement of people is expected to change due to CC (medium evidence, high agreement). CC can increase violent conflicts in the form of civil war by amplifying well-documented drivers of these conflicts such as poverty and economic shocks (medium confidence).
  • Livelihoods & poverty: Throughout the 21st century CC is expected to reduce economic growth, make poverty reduction more difficult and further erode food security (medium confidence).

Future Surface Temperature Scenarios

The following diagram shows how the Earth’s temperature has changed in the last ~110 years (Part A). Any area with insufficient data was left white and any statistically insignificant temperature change is shown with hatched lines. The second diagram in the figure below (B) shows two of the RCP scenarios and how the world’s average temperature could increase whilst following these scenarios. The blue is considered the ‘best case’ scenario where countries adopt a very strict reduction of greenhouse gas (GHG) emissions whereas the red is a ‘carry on as normal’ scenario. The third part to this figure (C) shows two plots much the same as in part A but the two RCP scenarios have been used to show how the Earth’s surface temperature can change depending on what mitigation techniques are adopted. The percentage temperature increase is taken from the average of the Earth’s temperature from 1986-2001. The key things to note from this figure is how dramatically different the future climate could be and how uneven the warming across the world is (for example, the Northern Hemisphere warms much more than the Southern Hemisphere).

: Observed and projected changes in annual average surface temperature. This  figure informs understanding of climate-related risks in the WGII AR5. It illustrates temperature  change observed to date and projected warming under continued high emissions and under  ambitious mitigation.    Technical details: (A) Map of observed annual average temperature change from 1901 to 2012,  derived from a linear trend where sufficient data permit a robust estimate; other areas are white.  Solid colors indicate areas where trends are significant at the 10% level. Diagonal lines indicate  areas where trends are not significant. Observed data (range of grid-point values: -0.53 to 2.50°C  over period) are from WGI AR5 Figures SPM.1 and 2.21. (B) Observed and projected future  global annual average temperature relative to 1986-2005. Observed warming from 1850-1900 to  1986-2005 is 0.61°C (5-95% confidence interval: 0.55 to 0.67°C). Black lines show temperature  estimates from three datasets. Blue and red lines and shading denote the ensemble mean and  ±1.64 standard deviation range, based on CMIP5 simulations from 32 models for RCP2.6 and 39  models for RCP8.5. (C) CMIP5 multi-model mean projections of annual average temperature  changes for 2081-2100 under RCP2.6 and 8.5, relative to 1986-2005. Solid colors indicate areas  with very strong agreement, where the multi-model mean change is greater than twice  the baseline variability (natural internal variability in 20-yr means) and ≥90% of models agree on  sign of change. Colors with white dots indicate areas with strong agreement, where ≥66%  of models show change greater than the baseline variability and ≥66% of models agree on sign of  change. Gray indicates areas with divergent changes, where ≥66% of models show change  greater than the baseline variability, but <66% agree on sign of change. Colors with diagonal  lines indicate areas with little or no change, where <66% of models show change greater than  the baseline variability, although there may be significant change at shorter timescales such as seasons, months, or days.

Observed and projected changes in annual average surface temperature. This
figure informs understanding of climate-related risks in the WGII AR5. It illustrates temperature change observed to date and projected warming under continued high emissions and under ambitious mitigation.

Future Fishing Scenarios

The final figure in this post shows the possible changes to fishing by the middle of this  century. The plot shows the redistribution of the maximum catch potential for over 1000 fish species caught worldwide, and is compared to the 2001-2010 baseline value. The values calculated here are based upon one of the more extreme RCP scenarios. Coastal regions and seas in north-western Europe and other mid- to high-northern latitudes will see an increase in maximum catch potential as more warm-water species migrate northwards. There will be a sharp decrease in maximum catch in the equatorial regions and around the South Pole with differences being as small as half the expected value of the 2001-2010 mean.


Climate change risks for fisheries. (A) Projected global redistribution of  maximum catch potential of ~1000 exploited fish and invertebrate species. Projections compare  the 10-year averages 2001-2010 and 2051-2060 using SRES A1B, without analysis of potential  impacts of overfishing or ocean acidification.

Climate change risks for fisheries. (A) Projected global redistribution of maximum catch potential of ~1000 exploited fish and invertebrate species. Projections compare the 10-year averages 2001-2010 and 2051-2060 using SRES A1B, without analysis of potential impacts of overfishing or ocean acidification.

 The IPCC WG2 also goes into detail about adaptation strategies however these have not been covered in this blog post. For more detail on this I suggest Section C of the Summary for Policy Makers. The third and final report by the Working Group 3 (WG3) has been published, and concentrates on potential mitigation choices – the subject of my next post.

A brief Introduction to Global Warming

Hello and welcome to my first blog post! I can’t really start without an explanation of how global warming occurs, as this is one of the most fundamental reasons why climate science exists! So here we go…

The Earth’s atmosphere allows life as we know it to exist. Without it, global temperatures would be around 33-35 oC colder. The gases that make up our atmosphere can absorb some of the sun’s energy leading to an increase in temperature. Energy from the Sun reaches the Earth in the form of light and heat. The Earth has the ability to absorb all of this radiation but some is reflected back into space either by the Earth’s surface or by its atmosphere. This process is called the Albedo Effect and without it the Earth would be much warmer. The Earth re-radiates energy at a slightly longer wavelength that can be absorbed by some atmospheric gases. These gases then re-radiate this as thermal energy.

A measure of how strong an effect this re-radiated energy has on the Earth’s temperature is called a gas’ radiative forcing. Radiative forcing is the difference between radiant energy received by the earth and energy radiated back into space, and its units are in watts per meter squared (Wm-2). The gases that contribute most to the greenhouse effect, in terms of their radiative forcing, their lifetimes and also their abundance in the atmosphere include carbon dioxide (CO2), water vapour (H2O) and methane (Ch4). The relationship between temperature and concentrations of these gases is strikingly positively correlated, as the graph below shows. In another blog I plan to explain more about how the Earth’s temperature has changed throughout geological history.

Record of the Earth's temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

Record of the Earth’s temperature and carbon dioxide concentrations taken from ice core data in Antarctica.

Almost all atmospheric gases have both natural and anthropogenic (man-made) sources. In fact, for most of the major greenhouse gases natural sources outweigh man-made. Since the Industrial Revolution, however, anthropogenic emissions have been rapidly increasing. The plots below, published by the Intergovernmental Panel on Climate Change (IPCC), show just how high the atmospheric concentrations have risen. It is this increase, or offset from the pre-industrial balance between sources and sinks, that is causing temperatures to rise. For more information on the IPCC and its soon to be released 5th assessment report click here.  Or if you want a more detailed summary of all the aspects of climate change this ‘summary for policy makers’ is really interesting.

Timeseries of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissionsfor the last 10,000 years.

Time series of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissions for the last 10,000 years.

Of course natural variability also affects the earth’s climate and can contribute to global warming. Examples of natural variability include variations in the Earth’s orbits (i.e. Milankovitch cycles – more to come in another blog), solar radiation variability, lunar tides and interactions between the oceans and the atmosphere. The world’s scientific community agree that the Earth is currently warming however there is a degree of discussion about how much human impact is contributing to global warming. The IPCC has stated that ‘the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’. Very likely being classed as a >90% probability.

Thanks for reading what I hope is the first of many blog posts.  Next up I will discuss changes to climate that occur on longer times scales, i.e changes to the Earth’s orbit about that sun.