 Mi yw Joanna House. I work at Bristol University. I'm currently a Leatherhoom Research Fellow at Bristol University with the Bristol Cabot Institute. My research is on land use change and greenhouse gas emissions. That's deforestation, afforestation, use of land for bioenergy. So there's all different types of emissions due to human activity. Fossil fuels account for the majority of emissions but also land use change, so basically deforestation but also all different kinds of activity on the land contribute to both emissions and sinks on carbon cycle. At the moment land use change, primarily deforestation accounts for about 10% of all CO2 emissions of fossil fuel account for most of the rest. Historically though if we look back to about from the pre-industrial period so from about the 1750s to today, then land use change accounts for about 30% of all emissions. So particularly when there were large changes and expansion of agriculture and fossil fuel use were still quite low and land use change emissions were much more significant. So over the whole the history of not the entire history of human emissions but really going back for the significant big changes in human activity then land use change is about 30%. Of course prior to the industrial revolution when we didn't have big use of fossil fuels then land use changes were the main source of emissions when you have very early expansion of agriculture. But the land is also a sink for carbon dioxide. So plants take up carbon dioxide when they grow and when emissions were quite low level from just from land use change before fossil fuels and the land was also able to respond to that higher concentration and take up carbon CO2, excess of CO2 in the atmosphere. So then the land was more or less in balance. Now fossil fuel emissions are so high and use change emissions are also still high and carbon dioxide is going off at such a rapid rate that the natural sinks for CO2, the ocean and the land can't respond fast enough to keep up with the emissions and therefore you've got rising CO2 in the atmosphere. When plants grow, when they take up carbon dioxide from the atmosphere and release oxygen and that process is known as photosynthesis, plants also have a respire, it's called autotrophic respiration when they do the reverse. So at night time they'll release CO2. During the day they take up CO2 and release oxygen. At night time they release a bit of CO2 but their net effect is still to take up CO2 and that provides the carbon that is the basis of the carbohydrates for plant growth. When those plants die and the organic matter decomposes and that's another type of respiration called heterotrophic respiration that also releases carbon dioxide to the atmosphere. Not all of the carbon respire, some of it goes into the soil and then you have long-term sinks and you have inert carbon in the soil that is very hard to decompose as they get very long-term carbon sinks in soils. The carbon cycle is very simple. It's about the cycling of carbon through natural systems, through plants, through soils, through the ocean and back out into the atmosphere. The carbon cycle, the carbon is constantly flowing between these different systems and large amounts of carbon moves all the time. But without anthropogenic influence those systems are roughly in balance if you look over long time periods. The plants, I mean the massive, the natural flows of carbon are massive so for example, global gross primary production which is actually the amount of carbon that plants take up before any respiration is about 120 gigatons of carbon per year. When you look at what emissions are due to land use change deforestation you're talking about one gigaton carbon per year. So the natural flows are massive and it's quite difficult picking out the net human impacts against this natural, very large background flow flux. So although the human emissions are much smaller than the natural fluxes the natural fluxes approximately are in balance and so they're not causing a net increase of carbon dioxide in the atmosphere The human emissions however are very rapid and the natural systems don't have time to respond to them and so you get a net imbalance of raised carbon dioxide concentrations in the atmosphere. In the earth's past, in and out of the ice ages the concentration of CO2 in the atmosphere ranged between about 180 parts per million 280 parts per million and it took thousands of years for it to change between those states. Differences now, it's gone up to 350 and even topping 400 parts per million on a single day basis and that's happened over a period of a couple of hundred years. Instead of a change that's happened over thousands of years within bounds that have remained quite similar in and out of the last few ice ages we've now gone up way above those bounds and the rate of change is very rapid. Residence time when people are discussing greenhouse gases is thought of as the amount of time it takes for a pulse of a greenhouse gas to be taken out of the atmosphere so if you emitted a pulse of carbon dioxide how long would it take for that pulse of carbon dioxide that you've added to the atmosphere not to be seen in the atmosphere anymore? With methane and with nitrous oxide greenhouse gases that's easier calculation to make and for methane it's about 14 years for that additional pulse to be removed from the atmosphere and for nitrous oxide it's about 114 years. For carbon dioxide that calculation's not so easy to make there's not a single residence time because there's a multitude of different processes that remove carbon dioxide from the atmosphere so for example the CO2 from the atmosphere dissolves in the surface of ocean and then that's turned over and taken into the deep ocean and really for an amount of CO2 to be completely removed from the atmosphere it has to be completely dissolved and go down into the deep ocean and then we're talking about geological timescales so hundreds of thousands of years whereas there are also very short processes that are taking up carbon dioxide from the atmosphere so for example that dissolution into the ocean surface and taking up by plants so about 65 to 80% of the carbon dioxide pulse that's put into the atmosphere will be removed within about 200 years the rest of it, the remaining 35% will take between 2 and 20 millenia to be completely removed from the atmosphere so roughly you have to think whatever we're doing today whatever CO2 is being emitted roughly a third of it is going to stick around essentially forever really when you consider it in our lifetime I have come across people saying that residence time for CO2 is 100 years and that's a common misconception actually I've heard from the policy community and I've even seen written in articles about climate change and I think that I'm not really sure where that comes from and I think it might be to do with when people are talking about the relative global warming potential of different greenhouse gases of different scales and often they look at the 100 year time scale and compare other gases to carbon dioxide on the 100 year time scale but that doesn't mean the residence time is 100 years a third of what we put up remains in the atmosphere for millenia I've not come across the argument that rising CO2 in the atmosphere is caused by ocean warming no although the ocean warming does reduce the solubility of CO2 in the surface ocean so it reduces the efficiency of the ocean sink in the same way that climate change also affects land sinks as vegetation and soils respond to warming and changes in precipitation land use emissions in the future are going to depend very much on what we do with land in terms of needing cropland that's going to be the biggest driver of land use change in the future is how much land do we need to go food to feed an increasingly rising population that is also becoming increasingly dependent on meat which requires large crop areas to produce animal feed it's very uncertain but how big that expansion of agricultural land will be but if we continue to have population rise and we can't increase yields to meet that population rise then we're going to have to continue to deforest more areas plant more crops so no current scenarios future scenarios that account for population growth and feeding human population have negative emissions from land use some of them do have large areas of afforestation that's driven by climate policy so you run these through models that model various human systems and if you impose a high carbon tax that leads to it being economically viable to a forest large area so you do have large areas of afforestation but land use emissions still remain positive I did a degree in environmental science and at the time climate change was a emerging important topic and I got very interested in it I was also very interested in bioenergy at the time as a means of providing fuel for impoverished people and also as a way of mitigating climate then I got involved in IPCC I was employed initially to assist one of the convening lead authors working on the IPCC, the Intergovernmental Panel on Climate Change Reports and I really enjoyed the whole process of synthesising science and then presenting it to the policy community and ever since then I've carried on working within climate change sites also I had quite a passion for rainforest and seeing the rate of destruction of rainforest and wanting to link that both to the dual purposes of climate mitigation and biodiversity protection by slowing down the rate of rainforest loss although I work on climate change because I work specifically from the land point of view I guess the key question is how we continue to feed a rising population and because land use change emissions are so dominated just by how much agricultural land we're going to need and how much of that land is taken from carbon rich rainforests or grown on peatlands which have very carbon rich soils so I mean the biggest question which is also quite a big question for humans is how we feed a rising population in the future Specific questions around land use is really about how we can protect as much natural forest area while continuing to feed that population so that would be the big question for me Sciences aren't always the best people to communicate because it's not necessarily how we're trained we're trained to do science and it's an increasing pressure on us to communicate but I think it's very important I think the IPCC is actually a very good system of being able to enable scientists to come together and do science and having people who are experts in pulling that together and being able to present it in a way that's useful to the policy community and to public and different kinds of organisations and bodies that want to use that kind of data Obviously I think engagement is important and I do speak to the policy community and I think that's great but most scientists don't have training to do that and I think it's really difficult to particularly talk into the media I think scientists aren't trained for that we've not gone into that profession because it's not what we're good at and so it's quite a big learning curve However, there's other people putting forward information to the media all the time that's incorrect and we need to be able to engage with that somehow The major way humans have an impact on climate is by burning fossil fuels and if we want to do something to reduce climate impacts then we have to reduce our fossil fuel burning that's the bottom line 90% of emissions come from fossil fuels and we could plant forests that takes some CO2 out of the atmosphere but really that's only taking out the CO2 that was put into the atmosphere from cutting forests in the first place The major thing we have to do is reduce our fossil fuel emissions but I think there's also other important things we need to consider which is about forest protection not having further emissions from forests and that's useful from a climate perspective and also biodiversity protecting different communities as well and other things that tie in with that for example looking at our personal diet and use of resources but really the key is finding an alternative for fossil fuels for providing energy that's technically and socially acceptable