Could Carbon Offsetting Help Get Us Back Below 400ppm?

Atmospheric carbon dioxide (CO2) levels are often discussed in climate science, but for many people, the numbers can feel abstract. Terms like “400 parts per million” are frequently mentioned in headlines, yet rarely explained in a way that connects to everyday decisions or actions.

However, understanding what this figure actually means is important because it represents one of the most significant indicators of long-term climate change. It also helps clarify if reducing emissions going forward is enough to bring CO2 levels back down, and if not, whether carbon offsetting and carbon removal could play a role in reversing the trend.

What does 400ppm CO2 actually mean?

Parts Per Million (PPM) is a unit of measurement for the concentration of gases in the atmosphere. In simple terms, 400ppm means that for every million air molecules, 400 of them are carbon dioxide. While that may sound small, CO2 is a powerful greenhouse gas, and even small changes in its concentration can significantly affect global temperatures over time.

According to data from global monitoring stations, atmospheric CO2 levels passed 400ppm in 2016 and have continued to rise since then. For context, pre-industrial levels (before large-scale fossil fuel use) were around 280ppm. This means atmospheric CO2 has increased by more than 40% due to human activity.

Why is 400ppm considered important?

The 400ppm threshold is not a formal safe limit, but it has become a symbolic marker because it reflects how far human activity has already altered the atmosphere. Climate scientists use CO2 concentration as a key driver of global temperature change, as higher concentrations trap more heat in the atmosphere, contributing to rising global temperatures, changing weather patterns, and increased climate risks.

The UK’s own climate assessments, including those from the Climate Change Committee, highlight that long-term stabilisation of global temperatures requires not just reducing emissions, but also limiting total atmospheric greenhouse gas concentrations over time. CO2 does not disappear quickly, and once emitted, a portion remains in the atmosphere for hundreds to thousands of years, meaning today’s emissions have long-lasting effects.

Can we realistically get back below 400ppm?

At present, most climate pathways focus on stabilisation rather than returning to pre-400ppm levels in the near term. The UK Met Office and other scientific bodies emphasise that reducing emissions is essential to limit further increases, but reversing atmospheric concentrations is significantly more challenging.

To bring CO2 levels back below 400ppm would require two things simultaneously. First, emissions falling to net-zero or net-negative globally, and then large-scale removal of CO2 already in the atmosphere.  This second requirement is where carbon removal technologies and natural carbon sinks become essential.

The role of carbon removal and offsetting

Carbon offsetting is often discussed in terms of balancing emissions, but its role is evolving as the focus shifts towards carbon removal as well as reduction. Traditional offset projects include reforestation and afforestation, peatland restoration, renewable energy investment, and energy efficiency improvements.

These approaches primarily aim to reduce or offset emissions over time. However, more recent attention has turned towards carbon removal technologies such as Direct Air Capture (DAC), which physically extracts CO2 from the atmosphere and stores it permanently. Although these technologies are still developing, they represent one of the few ways to actively reduce atmospheric CO2 concentrations rather than simply slowing their increase.

While technological carbon removal is important, natural systems remain one of the most scalable tools currently available. Forests, soils and peatlands already act as major carbon sinks, and in the UK, peatlands alone store an estimated 3 billion tonnes of carbon, making their protection and restoration a significant climate priority.

However, these systems are not unlimited and can become saturated, damaged or reversed through deforestation, land-use change, or climate impacts such as wildfire and drought. This is why long-term climate strategies often combine both natural and technological approaches.

Where carbon offsetting fits into the bigger picture

Carbon offsetting sits at the intersection of emissions reduction and carbon removal. While it does not directly lower atmospheric CO2 levels on its own, it helps fund and scale projects that contribute to both goals. In practical terms, offsetting can support renewable energy development that prevents future emissions, fund the restoration of ecosystems that act as carbon sinks, finance emerging carbon removal technologies, and provide a mechanism for addressing unavoidable short-term emissions.

It is not a substitute for cutting emissions at source, but it is part of a broader system that may eventually help slow, and potentially reverse, atmospheric CO2 growth. However, even if carbon removal technologies and offset projects continue to expand, scale remains the central challenge. Global CO2 emissions are measured in tens of billions of tonnes per year, whereas, in contrast, current carbon removal capacity is still relatively small. Bridging that gap will require sustained investment, technological development, and policy support, which is why most climate strategies prioritise emissions reduction first, with carbon removal and offsetting playing a complementary role.

Final thoughts

Atmospheric CO2 levels above 400ppm are a reflection of long-term changes driven by industrial activity and fossil fuel use. While reducing future emissions is essential, it is not, on its own, enough to quickly bring those levels back down. Carbon offsetting cannot reverse atmospheric CO2 concentrations on its own, but when linked with credible carbon removal approaches and large-scale emissions reductions, it becomes part of a wider system that could, over time, help shift the trajectory of atmospheric carbon.

At Offset to Zero, this is the space we focus on, supporting the kinds of projects and technologies that may shape a lower-carbon atmosphere in the future. Want to find out more? Start offsetting today!