However, outside the eurozone, the UK has seen its economy rebound quickly because it can adopt relaxed fiscal and monetary policies independently and make necessary readjustments. In particular, sustained and rapid growth of the service sector has become a major engine driving UK's economic recovery. In 2015, the UK may surpass France to regain the position of the world's fifth biggest economy.

It is true that emerging markets and developing countries as a whole are growing more rapidly than developed economies, but the level of growth among them is diverging.

African economies are growing robustly, while developing economies in Asia are still the fastest growing region in the world. According to the forecast by the IMF, Sub-Saharan Africa and Asia's developing economies will see growth rates of 5.8 percent and 6.6 percent respectively in 2015, higher than the average growth of 5 percent in emerging markets.

Development gaps among BRICS countries are becoming prominent: China and India still maintain rapid growth, which are expected to hit 7.1 percent and 6.4 percent respectively in 2015. Most international organizations expect the Chinese economy will perform well, holding that China is exploring ways to control its slowdown process with the aim to realize sustainable economic growth.

Brazil and Russia, however, will face more difficulties. Russia may see negative economic growth in 2015 due to declining oil prices and sanctions by the United States and Europe. Russian President Vladimir Putin estimates that in 2015 the country's economy will grow by 0.6 percent and will rebound in two years at most.

Twin engines

With steady recovery in developed countries and slower growth in emerging markets, the world economy resumes a twin-engine model. However, special attention must be paid to two countries--the United States and China.

The United States has made the boldest rebalancing efforts and achieved the best results, while China has created the best top-down design for comprehensively deepening reform. In the future, the world economic growth will be driven by developed countries led by the United States and emerging markets led by China.

According to the IMF, the Chinese and U.S. economies accounted for 13.4 percent and 22.4 percent of the world's total GDP in 2014, contributing 30.5 percent and 22.3 percent respectively to global economic growth. In 2015, the Chinese and U.S. economies will grow by 7.1 percent and 3.1 percent, the most rapid in emerging markets and major developed economies respectively. These two countries will contribute 23.6 percent and 22.2 percent to the world economic growth.

Therefore the negative impact brought by the sluggish economy in the eurozone and Japan will be under control only if the U.S. economy remains smooth. Although some emerging economies are experiencing recessions for the time being, emerging markets can avoid serious troubles, as long as China can maintain medium-high growth of around 7 percent. The demand from China will support bulk commodity prices from declining sharply, and China's investment can offset some of the impact of capital outflow caused by an interest rate hike in the United States. It is crucial that China makes steady progress, accelerates the reform and economic transformation and develops a low-carbon economy, which is the biggest contribution it can make to the world economy.

Risks can't be ignored

The world economy will face similar challenges as China does: the end of stimulus policies, painful restructuring, shift from medium-to-high growth to medium-to-low growth, high unemployment rates, high debt ratios, aging populations, high leverage but low demand, low prices, low inflation and low growth. These conditions were not present in any of the previous cyclical adjustments.

More importantly, it is impossible for all the economies to resume the growth pattern that preceded the economic crisis. The world economy has failed to introduce new impetus and new vitality amid potentially declining growth rates, swelling aging populations, and slowing productivity.

The most urgent issue in 2015 is that the Federal Reserve will raise the interest rate. A rate hike in the United States should be favorable to the world economy, but the continual quantitative easing by the Japanese and European central banks will force emerging markets into a dilemma.

The recent sharp fall of international oil prices have caused the depreciation of the ruble and inflation in Russia. Other emerging markets are also facing similar crises. Over the long run, a rate hike by the Federal Reserve may arouse various unexpected economic and financial chaos, with emerging markets being particularly vulnerable because of their internal fragility. This is the biggest risk the world economy will face. All countries must help each other to maintain the steady and sustainable growth of the world economy.

Since the global financial crisis, potential growth rates in both developed countries and emerging markets have dropped. According to the IMF, in 2003-08, the average potential growth rate in the world was near 4.5 percent, but the figure fell to 3-3.5 percent in 2010-13 and may be even lower in 2014-18.

Major reasons for the slowdown of potential growth rates include impacts of the financial crisis, such as high debt ratios, high unemployment rates and high fiscal deficits; aging populations, which have become a common issue faced by both developed countries and countries with emerging economies; and lack of technological breakthroughs in the short and medium term. All these unfavorable factors will encumber the global potential growth rate, and it will be unlikely for all countries to resume pre-crisis economic growth speeds.

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