Read the text on climate change and answer question

According to all measurements, climate change is happening. But scientists appears to be split on what to do about it. Unfortunately, scientists do not all agree about the causes of global warming. In a recent book, two scientists – Fred Singer, a climate physicist, and Dennis Avery, a biologist – argue that the warming currently observed around the world is part of a 1,500-year cycle in solar energy. Singer, an outspoken critic of the idea that humans are warming the planet, and Avery, believe that a well established,1,500 year cycle in the Earth’s climate can explain most of the global warming that has taken place in the last 100 years. We are currently in an upswing, getting back on the downswing, and getting colder again. They say that efforts to slow down the current warming by reducing emissions of greenhouse gases are at best pointless, or at worst economically damaging. This, of course, is not what the fourth assessment report of the UN Intergovernmental Panel on Climate Change (IPCC) said a few weeks ago. That report from UN climate science working group has concluded that it is likely that rising greenhouse gas concentrations have caused most recent warming and that, depending on our actions now to slow he growth of emissions, warming by 2100 will probably be between about 1.5ºC and 6º C. So, which scientists tell us the truth?

From: VINCE, Michael. Macmillan English Grammar in context. Oxford: Macmillan,2008. p 30.