With a growing multitude of smokestacks dismantled and polluting factories closed, China is scaling up its often painful efforts to curb greenhouse gas emissions and boost energy efficiency.

The Chinese government is currently deliberating the 12th Five-Year Plan for China's development from 2011-2015, a critical period for the country to meet its target set last year to reduce carbon dioxide (CO2) emissions per GDP unit by 40-45 percent by 2020 from the level of 2005.

"It's not easy to achieve it," Barbara Finamore, a senior member of the US-based environmental group National Resources Defense Council (NRDC), said earlier this month in the Chinese city of Tianjin on the sidelines of the latest round of UN climate change talks.

China has successfully slowed down its CO2 emissions growth in the past five years, but without continued efforts to improve energy efficiency and diversify its energy structure, it would only be able to reduce carbon intensity by 37 percent by 2020 from the 2005 level, added Finamore, director of the NRDC's China program.

Striking a similar tone, Fatih Birol, chief economist of the International Energy Agency (IEA), said China's development in the next five years will be crucial for meeting the challenges in energy and the environment.

Their cautious comments resonated with those of many Chinese experts and officials who note that after picking the low-hanging fruit in the initial stage, emission-cutting efforts will become inceasingly difficult in later phases.

In order to deploy the green technologies needed to achieve substantial improvements, China needs to make an additional investment of 1.9 trillion to 3.4 trillion yuan (US$285.7 billion to 511.3 billion) in the 12th Five Year Plan period, up from the 1.5 trillion yuan (US$225.6 billion) during the previous five years, according to a report released by McKinsey & Company, a global management-consulting firm.