The report, released on Wednesday, warns that countries must make substantial cuts to greenhouse gas emissions to limit global warming and reduce the risks associated with climate change.
Even under the most optimistic scenario, global average temperatures could rise by 1.7°C to 1.8°C by the middle of the century compared with pre-industrial levels.
According to the report, greenhouse gas emissions must fall 18 percent by 2030, 45 percent by 2035 and 69 percent by 2040, compared with 2025 levels, to limit global warming.
However, current national climate plans are expected to reduce emissions by only 5 to 8 percent by 2030 and 11 to 15 percent by 2035.
The UNEP report estimates that temperatures could still rise by 2.6°C by 2100 even if countries fully implement their existing climate commitments. Both projections are higher than the estimates published last year.
The report attributes the revised outlook partly to the United States' withdrawal from the Paris climate agreement and changes to previous estimates of greenhouse gas emissions.
As preparations continue for the UN climate conference COP31 in Türkiye, UN Secretary-General António Guterres has urged countries not to abandon efforts to limit global warming and prevent increasingly severe climate-related disasters, including floods and wildfires.
Guterres said efforts to limit the overshoot of the 1.5°C threshold must continue, adding that the extent of the overshoot could still be reduced.
Under the 2015 Paris Agreement, countries committed to keeping the rise in global temperatures well below 2°C above pre-industrial levels while pursuing efforts to limit it to 1.5°C.
However, UNEP says achieving the 1.5°C target has become increasingly difficult. Its projections suggest there is barely a 50 percent chance that temperatures could return to that threshold by 2100.
The report also notes that climate plans submitted since last year have made little difference to long-term temperature projections.
Returning global temperatures to the 1.5°C threshold would require removing large amounts of carbon dioxide from the atmosphere. However, uncertainty remains over whether this can be achieved on the scale required.
