Halliburton Co. also said its net income more than doubled as the oil services company benefited from an increase in drilling activity around the globe.

Net mums' co-founder Sally Russell said: "Research has shown that mothers are at their least happy stage in the first few years of bringing up children because they are so stressed and overworked.

"We could be seeing the first signs of net CO2 sequestration in the forest sector," said co-author Corinne Le quere.