Millions of people around the world are likely to be pushed back into poverty because climate change is undermining economic development in poor countries, the World Bank has warned.

Droughts, floods, heatwaves, sea-level rises and fiercer storms are likely to accompany increasing global warming and will cause severe hardship in areas that are already poor or were emerging from poverty, the bank said in a report.

Food shortages will be among the first consequences within just two decades, along with damage to cities from fiercer storms and migration as people try to escape the effects.

In sub-Saharan Africa, increasing droughts and excessive heat are likely to mean that within about 20 years the staple crop maize will no longer thrive in about 40% of current farmland. In other parts of the region rising temperatures will kill or degrade swaths of the savanna used to graze livestock, according to the report, Turn down the heat: climate extremes, regional impacts and the case for resilience.

In south-east Asia, events such as the devastating floods in Pakistan in 2010, which affected 20 million people, could become commonplace, while changes to the monsoon could bring severe hardship to Indian farmers.

Warming of at least 2C (36F) – regarded by scientists as the limit of safety beyond which changes to the climate are likely to become catastrophic and irreversible – is all but inevitable on current levels, and the efforts of governments are limited to trying to prevent temperature rises passing over this threshold. But many parts of the world are already experiencing severe challenges as a result of climate change, according to the World Bank, and this will intensify as temperatures rise.

Jim Yong Kim, the bank’s president, warned that climate change should not be seen as a future problem that could be put off: “The scientists tell us that if the world warms by 2C – warming which may be reached in 20 to 30 years – that will cause widespread food shortages, unprecedented heatwaves, and more intense cyclones.

“In the near-term, climate change – which is already unfolding – could batter the slums even more and greatly harm the lives and hopes of individuals and families who have had little hand in raising the Earth’s temperature.”

The development bank is stepping up its funding for countries to adapt to the effects of climate change, and is calling for rich countries to make greater efforts at cutting greenhouse gas emissions.

Rachel Kyte, vice president of the World Bank, said it had doubled its aid for adaptation from $2.3bn (£1.47bn) in 2011 to $4.6bn last year, and called for a further doubling. She said the bank was working to tie its disaster aid and climate change adaptation funding closer together.

Aid from the bank to help poor countries cut their greenhouse gas emissions and pursue environmentally sustainable economic development stands at about $7bn a year, and is backed by about $20bn from regional development banks and other partners.

The report’s authors used the latest climate science to examine the likely effects of global warming of 2C to 4C on agriculture, water resources, coastal ecosystems and fisheries, and cities, across sub-Saharan Africa, south and south-east Asia.

Kyte said the effects would be to magnify the problems that developing regions experience. More people would be pushed into slums, with an increased risk of disease. “We are looking at major new initiatives [in] cities; cities need billions of investment in infrastructure, but many developing cities are not really creditworthy,” she said.

She pointed to Jakarta, where rising sea levels and decades of pumping freshwater from underground sources beneath and around the city were increasing its vulnerability to flooding. Choices would need to be made soon in many cities on how to stem the likely effects, but Kyte warned that the plans must be future-proof, citing Ho Chi Minh City in Vietnam, which has been forced to rethink its flood preparations despite spending $2bn on them.

Green campaigners emphasised the need to try to avoid 2C of warming, which scientists stay is possible if countries bolster their ambitions to cut greenhouse gas ambitions in the near future.

Stephanie Tunmore, climate campaigner at Greenpeace International, said: “Fossil fuels are being extracted in burned in the name of development and prosperity, but what they are delivering is the opposite.

“Some major impacts from climate change are already unavoidable and rich countries must urgently support the poor and vulnerable to adapt. But massive increases in the future costs of adaptation and damage can only be avoided by investing in a clean energy future now.”

The World Bank has come under fire in the past for funding coal-fired power plants in some developing countries. However, it said the move was the result of old policies and was being phased out.

Britain’s offshore rigs and platforms have leaked oil or other chemicals into the North Sea on 55 occasions over the past month alone, challenging claims by the industry that it has a strong safety and environmental record.

Among the fields to have reported pollution discharges is Piper Alpha, the scene of the world’s worst offshore accident in terms of fatalities when it blew up, killing 167 workers, 25 years ago.

Facilities operated by Shell, BP and BG were all offenders, according to the latest petroleum operations notices (PON1s) reported to the Department of Energy and Climate Change (DECC).

Greenpeace said the alarming statistics should act as a reality check for an industry that was trying to persuade the world it should be allowed to drill in the pristine waters of the Arctic.

“They’re trying to convince the world that they can operate safely in one of the world’s harshest environments, yet they can’t prevent this steady trickle of oil and other polluting chemicals leaking into the relatively safe waters of the North Sea,” said Greenpeace senior climate adviser Charlie Kronick. “This will do little to increase public trust in their ability to drill in the Arctic without damaging this incredibly beautiful and fragile corner of our planet.”

But the industry itself says the leaks often contain tiny amounts of relatively harmless substances and the reporting system is an example of a good regulation.

One of the worst offenders in the latest set of DECC figures is Shell, which on 3 June reported lubricant and other chemical discharges from its Brent Bravo and Brent Charlie platforms.

A Shell spokesman said: “Asset integrity and process safety is Shell’s foremost priority at all times. No spill is acceptable and we work hard both offshore and onshore to minimise risks to maintain a safe working environment for our workforce and reduce any environmental impact on the North Sea.

“Shell is actively participating in the Step Change in Industry safety initiative, which includes a focus on hydrocarbon spill reduction. The industry has achieved an almost 50% reduction in hydrocarbons leaks during 2012, based on a baseline set in 2009.”

In 2006, Shell was fined £900,000 after pleading guilty to safety lapses on the Brent B platform following an accident in 2003, when the facility was hit by a gas leak in which two oil workers died.

BP, which is still fighting criminal charges following the Deepwater Horizon accident of 2010 in the US Gulf, is reported to have had crude leaks off the Paul B Loyd Jnr rig, which was working on the Clair field on 6 June this year. There was also a release of “another” substance from the same drilling unit two days earlier. On 25 May there was discharge on the Marnock field.

BG had a leak on the Everest North platform on 31 May while Talisman Energy discharged chemicals the day before on the Piper Bravo platform that was built in place of the Piper Alpha structure destroyed by fire in 1988.

Petroleum operations notices are all reviewed and investigated by an offshore environmental inspector as they are reports of potential breaches of DECC-enforced regulations.

Some of the discharges are allowable under North Sea rules but most on the latest PON1s monthly data whose status is marked “completed” rather than still “under review” ascribed the source to various mechanical failures.Those incidents that do show how much product was released indicate small amounts but any unintended action is unwelcome at a time when safety and the environment are major concerns of the public.

Although the PONS1 data seen by the Guardian for the month from 6 May to 6 June show 55 different numbered notices, employers dispute the figures and downplay their significance.

Mick Borwell, Oil & Gas UK’s environmental issues director, said: “The vast majority of the 103 spills this year [in PON1 reports] are very small operational chemical spills. They have no potential to cause a major accident, so do not compromise the increase in safety standards reported recently including a year on year reduction in combined fatal and major injury rates and in all types of dangerous occurrence and a 48% reduction in hydrocarbon releases over three years.”

BP and Shell declined to comment. On Monday, BP will appear in court in New Orleans to argue that the huge compensation package agreed last year following the Deepwater Horizon disaster is being abused. Lawyers for BP will claim that large numbers of “fraudulent, excessive or improper claims” are being filed to the victims’ fund, to which BP set aside around $8bn.

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