A first concern is the effectiveness of carbon offset schemes. Increasing environmental awareness among companies and individuals has sparked an emerging and lucrative market in carbon credits based on a plethora of carbon reduction projects. But due to a shortage of verification, the impact of these actions may be questioned. Some practices are inherently flawed, such as preventing clearcutting of forests that would be preserved for conservation anyway, or selling credits for cleaner and more efficient production techniques that are introduced for economic reasons. Other practices fail because of insufficient scientific understanding of the system.

For instance, (re)forestation in developing countries is one of the most popular offset schemes. Forests are a convenient method of capturing and storing atmospheric carbon. There are also several positive side effects. Many forests, in particular where native species are used, are a breeding ground for biodiversity. Forests may be planted on degraded areas and reduce erosion, and they may also provide a sustainable source of firewood and other environmental services for local inhabitants.

But there are negative side effects too. Carbon offset schemes tend to focus on fast-growing trees, such as eucalyptus and pine, which are non-native and have a far lower environmental and biodiversity benefit than native species. Tree plantations also tend to consume more water than grasses and shrubs. On a global scale, tree plantations decrease streamflow by about 200 mm per year (roughly 50%). This may have serious impacts on the local water cycle. For instance, extensive parts of the Andean páramo have been forested with pine for carbon sequestration purposes. The páramo is an ecosystem that consists of extensive grasslands in the upper parts of the tropical Andes, and stores vast amounts of water in its soils, swamps and lakes. It provides many environmental functions, but the most important is water supply for the highland area, including major cities such as Bogotá, Columbia, and Quito, Ecuador. Forestation with pine reduces streamflow by about 70%. Since most of this reduction affects low flow conditions, the consequences for local water security are serious.

What's more, the higher water consumption of pine may even result in a net carbon release to the atmosphere. Páramo wetlands store high amounts of organic carbon. This accumulation of organic matter is strongly linked to water saturation of the soils for large periods of the year. Dessication of the soils after forestation induces faster organic matter decomposition, which may offset carbon storage in the biomass above-ground. In Indonesia, the conversion of peat bogs into oil palm plantations has had similar effects.

Forestation activities in the Andes now focus more on biodiversity and use indigenous species such as Polylepis. However, there is no scientific consensus about the historical vegetation patterns of the páramo. What is considered reforestation may well be forestation of valuable and original grassland ecosystems. And since the impact of those forests on the local water cycle is not well understood, care should be taken not to disrupt a delicate and valuable hydrological system.

Even if such scientific questions are solved, significant concerns remain about the sustainability of carbon pools in forests. Forests capture most carbon early in their life cycle. The biomass of a mature forest is nearly stable, and organic carbon capture can only be maintained if carbon is transferred to another sink, such as the soil. As this is not always the case, organic carbon capture can only continue if both the current forest is maintained and ever more area is forested. Although this may seem preferable from an environmentalist viewpoint, it is not sustainable in the long term.

This lack of sustainability points to the essence of the problem. Our current carbon footprint is too high, and the only long-term solution is to reduce carbon emissions, not to compensate them by carbon offset schemes. This is also why carbon offset schemes are opposed from an ethical viewpoint. They are considered as paying someone else for reducing their greenhouse gas emissions and, as such, buying your way out of responsibility. And the schemes may distract attention from the real problem of how we reduce our own emissions. This is a double challenge for scientists. Finding new ways to decrease our society's greenhouse gas emissions is a major scientific challenge that merits our full attention. Considering whether the scientific value of a trip to the other end of the world outweighs the use of resources and the carbon footprint is a much more personal challenge.