Urban agriculture (UA) is more carbon-intensive than conventional agriculture, with food from UA sites having a carbon footprint six times higher than conventional agriculture (420 gCO₂e vs. 70 gCO₂e per serving). However, some UA crops and sites outperform conventional agriculture, suggesting that UA practitioners can reduce climate impacts by cultivating greenhouse-grown or air-freighted crops, maintaining sites long-term, and leveraging circularity. UA is intended to make cities more sustainable, but environmental claims are not well supported. Low-tech UA, which is more common, has been under-researched, with most studies focusing on high-tech forms. This study, using citizen science data from 73 UA sites in five countries, found that all forms of UA are more carbon-intensive than conventional agriculture, though some sites, like individual gardens, are more competitive. The carbon intensity of UA varies by country, with Poland having the lowest and the UK the highest. While some crops like tomatoes show lower carbon footprints in UA, this is often due to the carbon-intensive nature of conventional supply chains. To make UA more sustainable, practitioners should extend infrastructure lifespans, use urban waste as inputs, and maximize social benefits. The study highlights the need for policies that support stable land tenure and promote circular practices. Future research should explore seasonal dynamics, alternative supply chains, and better data on composting and fertilizer use to refine UA practices and reduce environmental impacts.Urban agriculture (UA) is more carbon-intensive than conventional agriculture, with food from UA sites having a carbon footprint six times higher than conventional agriculture (420 gCO₂e vs. 70 gCO₂e per serving). However, some UA crops and sites outperform conventional agriculture, suggesting that UA practitioners can reduce climate impacts by cultivating greenhouse-grown or air-freighted crops, maintaining sites long-term, and leveraging circularity. UA is intended to make cities more sustainable, but environmental claims are not well supported. Low-tech UA, which is more common, has been under-researched, with most studies focusing on high-tech forms. This study, using citizen science data from 73 UA sites in five countries, found that all forms of UA are more carbon-intensive than conventional agriculture, though some sites, like individual gardens, are more competitive. The carbon intensity of UA varies by country, with Poland having the lowest and the UK the highest. While some crops like tomatoes show lower carbon footprints in UA, this is often due to the carbon-intensive nature of conventional supply chains. To make UA more sustainable, practitioners should extend infrastructure lifespans, use urban waste as inputs, and maximize social benefits. The study highlights the need for policies that support stable land tenure and promote circular practices. Future research should explore seasonal dynamics, alternative supply chains, and better data on composting and fertilizer use to refine UA practices and reduce environmental impacts.