Answering from a California perspective, much, much lower.  The areas impacted first, the hot arid and desert parts of the planet like parts of India, are already being impacted by the Pakistan-Arabia Dust Cloud.  In the arid parts of the planet we will probably lose over 90% of the native species from 90% of their original ranges.  

I am not talking about complete extinction of species, just Spatial Extinction of large areas, which has already occurred over 99% of California for 99% of the native grassland biome species for example. 

As the climate in the already arid areas, gets hotter and drier very rapidly when the monsoon moisture is pushed further eastward by the Dust Cloud like this summer,  you will get an "Easter Island" effect, where the billion plus people will be stuck with nowhere to go, and the humans and their activities and needs will rapidly magnify the environmental impact of the new droughts.

Already the original native understory biomes in the subtropical and temperate parts of the planet are mostly gone, meaning everything that used to grow among the native grasses like wildflowers and forbs.  So the global warming change will be less grass to desert and barren areas like from North Africa to western India that you can see at http://www.ecoseeds.com/africa_india_megatransect.html.

OIr another example is my photo megatransect going from north to south through Pakistan and India, with 26 photos, and you only see the native understory (other than trees or shrubs) in only 5, and the rest is either cropland or desert. 

I am suggesting repairing the Dust Cloud issue in your area, and start replanting the Pseudomonas tropical forest host trees ASAP so your people do not end up like the Indus Valley Civilization thousands of years ago, at https://www.researchgate.net/post/New_group_to_discuss_monsoon_plus_Dust_Cloud_and_Pseudomonas_interactions .

The Pakistan to India photo megatransect is here http://www.ecoseeds.com/india_megatransect.html

Agree with Mr. Craig

Important higher plants may be replaced by woody climbers, lianas, shrubs. Introduced alien invasive species may promulgate more likely than native plants. Species may shift poleward or toward higher altitudes. Growth, regeneration, phenology, morphology may be changed. Growth and productivity may be increased or decreased, as focused by Vinesh. Due to shifting vegetation, corresponding feeding faunal diversity may also have to change their habitat, or otherwise decreased.

There is a significant relation between climate change and biodiversity.Today,climate change is one of the most important discussing factor in the world  which affects a lot to our environment via direct or indirect means. First of all there is decrease in the strength and fitness of the species. It affect the genetic diversity of a population due to directional selection and migration. Many species are geographically isolated just because of  habitat loss. New form of species may be arises via affecting  their resilience power. There is change in life cycle timing and food resources of a species also . Most of the species are extinct or in endangered category. So we can say that there is positive and negative impacts both of this climate change.

Hi THESES articles will help you:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB0QFjAA&url=http%3A%2F%2Fwww.biodiversitybc.org%2Fassets%2FDefault%2FBBC%2520Major%2520Impact%2520Climate%2520Change.pdf&ei=4PvBVKjbGIHrUNS9hMAI&usg=AFQjCNGcOeB0krXOknFuyDm4HZJIsfej_Q&sig2=ek5ha1cj-1diZHdF51YTMA&bvm=bv.84349003,d.d24

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCMQFjAB&url=https%3A%2F%2Fwww.cbd.int%2Fdoc%2Fbioday%2F2007%2Fibd-2007-booklet-01-en.pdf&ei=4PvBVKjbGIHrUNS9hMAI&usg=AFQjCNEh8TLabjfV0EnovCmbhAsLfsSAhg&sig2=x4aas3zIfDV8wQ6HCWHufw&bvm=bv.84349003,d.d24

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCMQFjAB&url=https%3A%2F%2Fwww.cbd.int%2Fdoc%2Fbioday%2F2007%2Fibd-2007-booklet-01-en.pdf&ei=4PvBVKjbGIHrUNS9hMAI&usg=AFQjCNEh8TLabjfV0EnovCmbhAsLfsSAhg&sig2=x4aas3zIfDV8wQ6HCWHufw&bvm=bv.84349003,d.d24

The benefit or losses can not be measured or estimated, however replacement or depletion of some species  may have benefits or losses for other species which may directly or indirectly influence in the form of benefit somewhere and loss somewhere.

It depends what climate change entails. If temperatures increase and there is plenty of rainfall, mega-biodiverse tropical rain forests may extend from the tropics into what are now temperate areas, thus increasing biodiversity. See paleontologist Carlos Jaramillo's papers: http://www.stri.si.edu/english/scientific_staff/staff_scientist/scientist.php?id=43

He studies the first modern rain forests in South America, home to Titanoboa, a snake as long as a city bus. When Titanoboa lived, accompanied by giant crocs and turtles at a time 60 million years ago, temperatures were significantly higher than they are now.

However, given the fact that humans continue to fragment forests into tiny pieces, thus disrupting biological connections, simply expanding the range of tropical forests does not guarantee increases in biodiversity. Bill Laurance can tell you more about that: http://www.jcu.edu.au/mtb/staff/az/JCU_127342.html

Also, if rainfall patterns change, biodiversity could decline precipitously, as tropical forests are very sensitive to drought. See Condit et al on Ben Turner's page: http://www.stri.si.edu/english/scientific_staff/staff_scientist/scientist.php?id=41

We're living the experiment. As Matt Larsen (director, Smithsonian Tropical Research Institute) said yesterday at a Smithsonian Conservation Commons meeting: "I don't have any tattoos, but if I were to get one, it would be The Keeling Curve."

Natural systems are too connected to each other that increase or decrease in any parameter may have profound effects on global phenomenon which sometime reverse or stabilize again to maintain the life system so the increase may be very small or short term to have very less effects on living systems but may continuous  to let life forms to be adapted somehow. 

Dear Vinesh

Check very interesting and informative responses of RG friends on a related threads

https://www.researchgate.net/post/How_can_we_protect_and_preserve_our_natural_resources

https://www.researchgate.net/post/Why_protection_and_preservation_of_biodiversity_is_a_low_priority_sector_compared_to_infrastructure_communication_space_and_defense_research

Dear Vinesh

I am attaching an interesting news item that appeared in a local newspaper in Hyderabad about the ill effects of climate change on forests in 2 provinces and the butterfly diversity and population density in a specific location.