Since the 1990s, the proportion of the world's population who are overweight (with a body mass index of 25-30) or obese (with a body mass index of 30 or above) has doubled.

If current patterns continue, we estimate that by 2050, 30% of the world's children and adolescents (aged five to 24 years) will be overweight or obese, according to our new research in The Lancet.

By 2050, we forecast that 2.2 million Australian children and adolescents will be living with obesity. A further 1.6 million will be overweight. This is a combined prevalence of 50% - and an increase of 146% between 1990 and 2050.

Already in 2017-18, excess weight and obesity cost the Australian government A$11.8 billion. The projected disease burden will add billions of dollars to these health costs.

So how did we get here? And most importantly, what can we do to turn this trajectory around?

Living with obesity increases the likelihood of living with disability and dying at a young age.

Obesity doesn't just cause health problems later in life. Living with obesity increases the chance of developing many serious diseases during childhood or adolescence, including fatty liver disease, type 2 diabetes and hypertension (high blood pressure).

Due to weight-related teasing, bullying and stigma, obesity can also cause problems with mental health, and school and community engagement.

Some of the negative health effects of obesity can be reversed if young people return to a normal weight.

But reducing your weight from an obese BMI (30-plus) to a normal weight BMI (18.5-25) is very difficult. As a result, 70-80% of adolescents with a BMI of 30 or above live their adult years with obesity.

So it's important to prevent obesity in the first place.

Obesity is often blamed on the individual child, parent or family. This is reflected in significant weight-based stigma that people in larger bodies often face.

Yet the rapidly changing patterns of obesity throughout the world reinforce the importance of viewing it as a society-level problem.

The drivers of the obesity epidemic are complex. A country's increasing obesity rates often overlap with their increasing economic development.

Economic development encourages high growth and consumption. As local farming and food supply systems become overtaken by "big-food" companies, populations transition to high-calorie diets.

Meanwhile, our environments become more "obesogenic", or obesity-promoting, and it becomes very difficult to maintain healthy lifestyles because we are surrounded by very convenient, affordable and addictive high-calorie foods.

Obesity arises from a biological response to living in these environments.

Some people are more negatively affected by living in these environments and gain more body weight than others. As our recent study showed, compared to those born with low genetic risk, adolescents who are born with a high genetic risk of developing obesity are more likely to become overweight or obese when living in poverty.

Other research shows those with a high genetic risk are more likely to gain weight when living in obesity-promoting environments.

The steepest increase in the proportion of young people with obesity is expected to be in the coming years. This means there is an opportunity to address this public health issue through bold actions now.

Some young people with severe obesity should be provided access to funded, stigma-free team-based weight-management health care. This may include:

access to GPs and nurses for lifestyle advice about diet and exercise

anti-obesity medications such as semaglutide

weight-loss surgery.

But to reach all young people, it is the overarching systems, not people, that need to change.

Success will be greatest if policies change multiple parts of the environmental systems that young people live in, including schools, food systems, transport systems and built environments. These changes will also reach older adolescents, whose rate of obesity continues to increase.

It is also important to target the commercial determinants of obesity. Strategies could include:

changes to food marketing and advertising policies, such as banning manufacturers from marketing junk food as health food

adding taxes on unhealthy foods, such as adding a levy to the cost of soft-drinks and other sugar-sweetened beverages

funding healthy school meals or canteen options.

This should be coupled with changes to the built environment and urban planning, such as increasing green space, footpaths and walkability.

Because obesity doesn't belong to any one part of government, action can fall through the cracks. Although there are significant efforts being made, action requires coordinated investments from numerous government portfolios - health, education, transport, urban planning - at local, state and national levels.

Governments should commit to an immediate five-year action plan to ensure we don't fail another generation of children and adolescents.