China is on course to exceed forecasts for greenhouse gas emissions because its economy is growing faster than expected and becoming "locked in" to carbon-intensive activities, two studies warn this week.

The reports cast new light on the enormity of the challenge faced by the world's biggest emitter, despite its impressive investments in renewable energy, clean technology and the efficiency of its power plants.

Researchers from Climate Action Tracker praised China for a "major effort" to meet and possibly surpass its Cancún pledge to reduce the carbon intensity of its economy by 40-45% by 2020.

But they said the gains were being outweighed by a faster-than-expected surge of the country's economy. By 2020, they predicted the net outcome of this "China emissions paradox" would be one gigatonne (GT) more carbon dioxide per year than the researchers' previous estimate of 12.5GT.

The report compared China's actions favourably with those of the US, which was criticised for a "lack of significant effort", but the authors said their new data showed the weakness of previous estimates and the need for urgent action.

"The sooner the switch is made to cleaner energy sources, the better, in order to avoid locking in climate-damaging energy," said Bill Hare of Climate Analytics.

A separate study suggested this may already be happening due to structural changes in the Chinese economy, which are creating new sources of greenhouse gas emissions.

Despite the world's focus on China's coal-fired power plants, a bigger threat may be the construction industry, warn the authors of Carbonising the Dragon, which was published on Wednesday in the journal Environmental Science and Technology.

The paper notes that China has been building the equivalent of 2bn square metres of floor space every year, which increases the country's carbon footprint enormously. Calculated at the point of consumption, cement is said to account for 46% of China emissions, and iron and steel 20% in total.

In contrast, it says electricity generation – which is usually cited as the primary problem – is about 30%. Despite the surge in demand for extra power, the authors say the Chinese government's investments in renewables and nuclear power have not greatly changed the energy mix.

Based on their study of data from 1992 to 2007, the paper concludes that China moved at the end of the period on to a more carbon intensive track.

"With the recent shift of China's economic growth away from export demand and towards capital investments, production is shifting towards energy and pollution intensive industries," it says.

Since 2007, China has attempted to reverse this trend, but the paper suggests more needs to be done to ensure buildings and infrastructure are designed with sustainability and low-carbon priorities in mind.