Massive protests unfolded in Glasgow outside the United Nations climate summit last week, with some activists denouncing a proposal to expand the use of a controversial climate action measure to meet net-zero targets: carbon offsetting.Offsetting refers to reducing emissions or removing carbon dioxide (CO2) from the atmosphere in one place to balance emissions made in another. So far, more than 130 countries have committed to the net zero by 2050 goal, but none is proposing to be completely emissions free by that date all are relying on forms of offsetting.The use of offsets in meeting climate obligations has been rejected by climate activists as a scam. Swedish climate campaigner Greta Thunberg, joining the protesters, claimed relying on buying offsets to cut emissions would give polluters a free pass to keep polluting.Others, however, argue offsetting has a legitimate role to play in our transition to a low-carbon future. A recent report by Australia's Grattan Institute, for example, claimed that done with integrity, carbon offsets will be crucial to reaching net zero in sectors such as agriculture and aviation, for which full elimination of emissions is infeasible.So who's in the right? We think the answer depends on the kind of offsetting that is being employed. Some forms of offsetting can be a legitimate way of helping to reach net zero, while others are morally dubious.The debate over offsetting is part of a key agenda item for COP26 establishing the rules for global carbon trading, known as Article 6 of the Paris Agreement. The trading scheme will allow countries to purchase emissions reductions from overseas to count towards their own climate action.To examine carbon offsetting in a moral context, we should first remember what makes our contributions to CO2 emissions morally problematic.The emissions from human activity increase the risks of climate change-related harms such as dangerous weather events storms, fires, floods, heatwaves, and droughts and the prevalence of serious diseases and malnutrition.The more we humans emit, the more we contribute to global warming, and the greater the risks of harm to the most vulnerable people. Climate change is a moral issue because of the question this invites on behalf of those people:Why are you adding to global warming, when it risks harming us severely?Not having a good answer to that question is what makes our contribution to climate change seriously wrong.The moral case in favour of offsetting is it gives us an answer to that question. If we can match our emissions with a corresponding amount of offsetting, then can't we say we're making no net addition to global warming, and therefore imposing no risk of harm on anyone?Well, that depends on what kind of offsetting we're doing. Offsetting comes in two forms, which are morally quite different.The first kind of offsetting involves removing CO2 from the atmosphere. Planting trees or other vegetation is one way of doing this, provided the CO2 that's removed does not then re-enter the atmosphere later, for example as a result of deforestation.Another way would be through the development of negative emissions technologies, which envisage ways to extract CO2 from the atmosphere and store it permanently.The second form is offsetting by paying for emissions reduction. This involves ensuring someone else puts less CO2 into the atmosphere than they otherwise would have. For example, one company might pay another company to reduce its emissions, with the first claiming this reduction as an offset against its own emissions.Australia's Clean Energy Regulator issues Australian Carbon Credit Units for eligible offsets projects. These include for projects of offsetting by emissions reduction.The regulator certifies that a company, for example, installing more efficient technology deliver abatement that is additional to what would occur in the absence of the project. Another company whose activities send CO2 into the atmosphere, such as a coal-fired power station, can then buy these credits to offset its emissions.There is a crucial difference between these two forms of offsetting. When you offset in the first way taking as much CO2 out of the atmosphere as you put in you can indeed say you're not adding to global warming.That's not to say even this form of offsetting is problem-free. It's crucial such offsets are properly validated and are part of a transition plan to cleaner energy generation compatible with everyone reaching net zero together. Tree-planting cannot be a complete solution, because we could simply run out of places to plant them.But when you offset in the second way, you cannot say you're not adding to global warming at all. What you're doing is paying someone else not to add to global warming, while adding to it yourself.The difference between the two forms of offsetting is like the difference between a mining company releasing mercury into the groundwater while simultaneously cleaning the water to restore the mercury concentration to safe levels, and a mining company paying another not to release mercury into the groundwater and then doing so itself.The first can be a legitimate way of negating the risk you impose. The second is a way of imposing risk in someone else's stead.Let's use a few simple analogies to illustrate this further. In morality and law, we cannot justify injuring someone by claiming we had previously paid someone who was about to injure that same person not to do so.The same is true when it comes to the imposition of risk. If I take a high speed joyride through a heavily populated area, I cannot claim I pose no risk on people nearby simply because I had earlier paid my neighbour not to take a joyride along the same route.Had I not induced my neighbour not to take the joyride, he would've had to answer for the risk he imposed. When I do so in his place, I am the one who must answer for that risk.In our desperate attempt to stop the world warming beyond the internationally agreed limit of 1.5 degree Celsius, we need to encourage whatever reduces the climate impacts of human activity. If selling carbon credits is an effective way to achieve this, we should do it, creating incentives for emissions reductions as well as emissions removals.What we cannot do is claim that inducing others to reduce emissions gives us a moral license to emit in their place.