Since assuming office, President Donald Trump has withdrawn the US from its international commitments to reduce carbon emissions and fight climate change under the Paris Agreement, rolled back domestic environmental legislation, and questioned climate change science, suggesting in an October interview with Fox News that climate scientists have a “political agenda”.

During a cold snap in late November, he joked on Twitter, “Whatever happened to global warming?” The president does not take climate change seriously.

But the US government does.The fourth national climate assessment, conducted by 13 federal government agencies, provides a sobering and, at well over 1,000 pages, long look at the impact of climate change on Americans lives and livelihoods. And its conclusions could not be clearer. The report warns that:

The evidence of human-caused climate change is overwhelming and continues to strengthen, that the impacts of climate change are intensifying across the country, and that climate-related threats to Americans’ physical, social, and economic well-being are rising.

Responding to this chilling report by his own government, Trump simply responded that “I don’t believe it”. He really should. To save you reading the full document, here are some of the most important points.

Climate change could knock off over 10 per cent of GDP

Under a worst-case scenario, under which temperatures rise by 9°F (5°C) by the end of the century, climate change could cost the US over 10 per cent of its GDP by 2090. This includes $155bn per year in lost labour costs due to extreme heat, $141bn per year lost as a result of deaths due to extreme temperatures, $118bn in damages due to coastal flooding and $9bn a year due to disruptions to the electricity supply. You can read more about the economic impact here.

More Americans will die because of poor air quality

Already, more than 100m Americans live in communities where air pollution exceeds health standards. An increase in atmospheric pollutants will lead to more deaths due to cardiovascular illnesses and aggravated asthma, the report forecasts. Allergies such as hay fever are also likely to get worse, due to a longer pollen season and because dryer, hotter air will carry more allergens.

Americans’ mental and physical health will suffer

More people will die annually because of extreme heat, especially in urban areas, which are often hottest. The number of heatwaves in the US has already tripled between 1960 and 2010. Hotter temperatures mean that mosquito-borne illnesses such as Zika and Dengue fever will travel north. As temperatures rise, it will be harder to maintain food safety or a safe drinking water supply. Rates of mental illness are also likely to increase, particularly in the aftermath of destructive and deadly weather events such as hurricanes or floods. Hotter temperatures have also been shown to increase aggression and lead to higher murder rates.

Poor and marginalized communities will be hardest hit

The impact of climate change varies dramatically across the US. Along the coast, where 42 per cent of Americans live, rising sea levels and fiercer storms threaten $1trn of national wealth and the continued viability of coastal communities. Throughout the country, poorer Americans will be hardest hit, and social inequality is likely to rise. Indigenous communities are also particularly vulnerable to changes in the climate.

Climate change threatens US food security

Yields of major US crops, such as corn, soy, wheat, rice and cotton, are expected to decline over this century because of rising temperatures as well as possible changes in water supply, and disease and pest outbreaks. Climate change is also expected to lead to large shifts in the availability and prices of many agricultural products around the world, which will impact US food producers and the US economy too.

Expect extreme weather events – and extreme uncertainty

Hurricanes, floods, droughts and other extreme weather events are expected to become more common. Wildfires are burning more than 10m acres of forests a year, compared to 2m in the 1980s. In 2017 alone, the Atlantic hurricane season is estimated to have caused more than $250bn in damages and over 250 deaths. An extreme drought across the Northern Great Plains caused $2.5bn in agricultural damages.

The report notes that, since 2014, when the third national climate report was published, scientists have improved their understanding of how climate change contributes to extreme weather conditions. But there’s another word of warning. Sometimes, when long term shifts in climate reach a tipping point, the consequences can be dramatic and unpredictable. “The more the climate changes, the greater the potential for these surprises,” the report warns.

But don’t despair yet

The worst-case scenarios can be avoided. Mitigating the effects of climate change, by reducing carbon emissions, could save billions of dollars in annual damages due to climate change, and adapting to a changing climate can shield Americans from its worst effects.

What the report doesn’t discuss is that this requires a US administration that is serious about climate change and willing to think long term and act in the public interest. Unfortunately, that’s not Trump.