How Climate Change Affects Human Health: Evidence from the 2021 Lancet Countdown

Study in one sentence

The 2021 Lancet Countdown report found that climate change is increasingly harming human health worldwide, with rising heat-related deaths, expanding infectious disease risks, and growing food insecurity, while the global response remains dangerously inadequate.

Publication and date

This study was published as “The 2021 report of the Lancet Countdown on health and climate change: code red for a healthy future” in The Lancet, a leading peer-reviewed medical journal, in October 2021. The lead author is Marina Romanello, and the report represents the work of over 100 experts from 38 academic institutions and UN agencies.

Research question

The Lancet Countdown is an annual international collaboration that monitors the evolving health profile of climate change. The 2021 report asks: How is climate change affecting human health today, and how adequate is the global response? It tracks progress across five key domains: climate change impacts, exposures, and vulnerability; adaptation, planning, and resilience for health; mitigation actions and health co-benefits; economics and finance; and public and political engagement.

Method

The report uses 44 indicators to assess the links between climate change and health. Data are drawn from a wide range of sources, including satellite observations, ground-based measurements, epidemiological studies, and economic models. The indicators are updated annually where possible, and new indicators are added to capture emerging threats. The analysis covers global and regional trends, with a focus on the period from 2000 to 2020. The methods are peer-reviewed and follow a standardized protocol to ensure consistency and comparability across years.

Main findings

The 2021 report documents several alarming trends:

• Heat-related mortality: Between 2000–2004 and 2017–2021, the number of heat-related deaths in people older than 65 increased by 53.7%, reaching an estimated 296,000 deaths in 2018 alone.
• Infectious disease transmission: The number of months suitable for malaria transmission in highland areas of the Americas, Africa, and the Asia-Pacific region increased by 39.1% for Plasmodium falciparum and 33.6% for Plasmodium vivax from 1950–1954 to 2012–2016. The climate suitability for dengue transmission rose by 12% over the same period.
• Food insecurity: In 2019, an additional 2 billion person-days of heatwave exposure occurred among the over-65 population compared with the 1986–2005 baseline, affecting crop yields and labor capacity.
• Health system preparedness: Only 0.3% of global climate finance was directed to health systems in 2019–2020, and just 51% of countries reported having a national health and climate change strategy.
• Economic costs: The economic cost of heat-related mortality among people aged 65 and older was estimated at $1.4 trillion in 2018, equivalent to 1.8% of global gross domestic product in that year.

What the findings do not prove

The report identifies associations and trends but does not establish direct causation for every health outcome. Many of the indicators are based on modeling and observational data, which can show correlations but cannot definitively prove that climate change alone caused a specific health event. The findings do not predict future health impacts with certainty; they describe current and past conditions and project potential risks under different scenarios. The report also does not claim that all observed health effects are solely due to climate change, as other factors such as socioeconomic development and public health infrastructure play significant roles.

Limitations

The study has several limitations. Data availability is uneven across regions, with low-income countries often underrepresented, which may skew global estimates. Many indicators rely on modeling that involves assumptions and uncertainties. The report tracks a limited set of health outcomes and does not capture all climate-sensitive diseases or indirect effects. Additionally, the indicators are proxies and may not fully reflect the complexity of health impacts. The analysis is constrained by the time lag in data reporting, meaning the most recent data may already be outdated.

How it compares with previous research

The findings are consistent with earlier Lancet Countdown reports and with assessments from the Intergovernmental Panel on Climate Change (IPCC). The 2021 report shows a continued worsening of health impacts compared with previous years, particularly in heat-related mortality and infectious disease suitability. It also places greater emphasis on the health co-benefits of climate action, such as reduced air pollution from phasing out fossil fuels, aligning with a growing body of research highlighting the immediate health gains from mitigation policies.

Why it matters

This report provides the most comprehensive and up-to-date evidence that climate change is a health emergency. It underscores the need for health to be central in climate policy, from national adaptation plans to international finance. The findings are directly relevant to policymakers, health professionals, and the public, offering a clear call to action: the same actions that reduce greenhouse gas emissions can also yield substantial health benefits, such as cleaner air and healthier diets. The report’s indicators serve as a benchmark for tracking progress and holding governments accountable.

Link to the primary paper

Romanello M, McGushin A, Di Napoli C, et al. The 2021 report of the Lancet Countdown on health and climate change: code red for a healthy future. Lancet. 2021;398(10311):1613-1662. doi:10.1016/S0140-6736(21)01787-6.